Codes:

Try removing this line:
Instead, install the new version of New Summer, the one with a .pkg file extension which needs mod installer by daeman. See thread here

I've already got the latest one, but I re-installed it, just to see what happens
UPDATE: I installed the Honda Civic Si and it works. Touch wood this is an one off event.

Even though you think you installed the mods right... you didn't. i am just telling you that you did not put the codes in right. trust me.

<?xml version="1.0" encoding="UTF-8" ?>

<Cars>




<!--
How to add a new car.


I. Adding a new car to menu


Following is a list of parameters, which should be specified for a new car section:

'Name' attribute - this attribute is your car identificator. All gfx and settings for cars are situated in folders
named as their respective car ID's. For example, a 'Car28' car have all its data placed in following folders:

data\gui\Common\layouts\cars\Car28\
data\gui\3di_home\imagesets\cars\Car28\
data\gamedata\cars\Car28\
data\physics\cars\Car28\
data\physics\Engine\Car28\
data\physics\Transmission\Car28\
export\anims\cars\Car28\
export\gfxlib\cars\Car28\
export\meshes\cars\Car28\
export\texturesdds\cars\Car28\

All car identifiers SHOULD BE UNIQUE. Try to create an unique and distinctive IDs so you wouldn't collide
with other user cars and future developer cars.

'ABS' attribute - this attribute defines if this car could be driven with an antilock brake system.
Values: 'true' - ABS is enabled, 'false' - ABS is disabled.

'AT' attribute - this attribute defines if this car could be driven with an automatic gearbox.
Values: 'true' - automatic gearbox is enabled, 'false' - automatic gearbox is disabled.
NOTE that all cars should have a default manual gearbox.

'DisplayName' element - this element contains a name for the car, which is displayed in the menu.

'Description' element - this element contains a description for the car, such as technical specifications,
car creator or whatever else.

All listed parameters ARE MANDATORY.

AN IMPORTANT NOTE: This file should be saved with UTF-8 encoding.


II. Defining an image for a new car.

one should add all necessary image files to the following path:

data\gui\3di_home\imagesets\cars\<Car ID>\

An examples of such files could be found at

data\gui\3di_home\imagesets\cars\Car01\


III. Example of new car description.

<Car Name="Shelly5000" ABS="false" AT="false" >
<DisplayName>Shelly Warsteed GT 5000</DisplayName>
<Description>
Shelly Warsteed GT 5000

Rear wheel drive
Manual transmission, 6 gears
Engine power of 5000 hp
Engine displacement 3000 cm3
Maximum speed 1000 km/h

Have your helmet on so it could be easier to pick up your head.
</Description>
</Car>

-->

<!-- ADD NEW CARS BELOW THIS LINE -->

<Car Name="LR_RangeRover_12" ABS="true" AT="true" >
<DisplayName>Range Rover 2012</DisplayName>
<Description>
Range Rover 2012
Drive Type: AWD
Type of gearbox and number of gears: automatic 6
Engine power, BHP: 313
Capacity, cc: 4367
Max Speed: 214 km/h
</Description>
</Car>
<Car Name="fiesta" ABS="true" AT="true" >
<DisplayName>Ford Fiesta</DisplayName>
<Description>
Ford Fiesta
Drive Type: Front wheel drive
type of gearbox and number of gears: Manual 5, Automatic, 4
Engine power, BHP: 100
Capacity, cc: 1000
Max Speed: 195 km/h
</Description>
</Car>
<Car Name="VOL_XC70_12" ABS="true" AT="true" >
<DisplayName>Volvo XC70</DisplayName>
<Description>
Volvo XC70
Drive Type: AWD
type of gearbox and number of gears: Automatic, 5
Engine power, BHP: 163
Capacity, cc: 1984
Max Speed: 196 km/h
</Description>
</Car>
<Car Name="qashqai" ABS="true" AT="true" >
<DisplayName>Nissan Qashqai</DisplayName>
<Description>
Nissan Qashqai
Drive Type: AWD
Transmission type: CVT
Engine power, hp 150
Capacity, cc: 1996
Max. speed, km / h: 194
</Description>
</Car>
<Car Name="smart_fortwo" ABS="true" AT="true" >
<DisplayName>Smart Fortwo</DisplayName>
<Description>
Smart Fortwo
Drive Type: Rear
Type of gearbox and number of gears: Manual - 5
Engine power, BHP: 52
Capacity, cc: 659
Max Speed: 135 km/h
</Description>
</Car>
<Car Name="CAD_Escalade_ESV" ABS="true" AT="true" >
<DisplayName>Cadillac Escalade ESV</DisplayName>
<Description>
Cadillac Escalade ESV
Drive Type: Rear / AWD
Type of gearbox and number of gears: Automatic - 5
Engine power, BHP: 409
Capacity, cc: 6199
Max Speed: 250 km/h
</Description>
</Car>
<Car Name="santa_fe" ABS="true" AT="true">
<DisplayName>Hyundai Santa fe</DisplayName>
<Description>
Hyundai Santa fe 2.4
Drive type: 4wd
Type of gearbox and number of gears: mechanic - 5/auto - 6
Maximum power: 175 HP on 5600 RPM
Engine displacement: 2398 cc
Maximum speed: 215 km/h
</Description>
</Car>
<Car Name="SUB_For_by_trafim" ABS="true" AT="true" >
<DisplayName>Subaru Forester</DisplayName>
<Description>
Subaru Forester
Drive type: AWD
Type of gearbox and number of gears: mechanic - 5 automatic - 4
Maximum power: 230 HP
Engine displacement: 2547 cc
Maximum speed: 250km/h / 207 km/h
</Description>
</Car>
<Car Name="VolvoXC90_by_Daeman" ABS="true" AT="true" >
<DisplayName>Volvo XC90</DisplayName>
<Description>
Volvo XC90
Drive type: AWD/rear
Type of gearbox and number of gears: mechanic - 6, automatic - 5
Maximum power: 238 HP on 6200 RPM
Maximum torque: 320 N*m on 3200 RPM
Engine displacement: 3192 cc
Maximum speed: 210 km/h
</Description>
</Car>
<Car Name="bmw_7_11" ABS="true" AT="true" >
<DisplayName>BMW 7-series 2011</DisplayName>
<Description>
BMW 7-series 2011
Drive type: full
Type of gearbox and number of gears: mechanic - 5; automatic - 5
Maximum power: 449 HP on 5500 RPM
Maximum torque: 650 N*m on 3800 RPM
Engine displacement: 4.4 litres
Maximum speed: 250 km/h
</Description>
</Car>
<Car Name="ford_focus_st_2006_1.6" ABS="true" AT="true" >
<DisplayName>Ford Focus 1.6</DisplayName>
<Description>
Ford Focus 1.6 2006
Drive type: FWD
Type of gearbox and number of gears: Manual: 6; Auto: 4
Maximum power: 100 HP @ 6000 RPM
Engine displacement: 1596 cc
Maximum speed: Manual: 180; Auto: 172 km/h
0-100 km/h: Manual: 11,9; Auto: 13,6 sec
</Description>
</Car>
<Car Name="ford_focus_st_2006" ABS="true" AT="false" >
<DisplayName>Ford Focus ST</DisplayName>
<Description>
Ford Focus ST 2006
Drive type: FWD
Type of gearbox and number of gears: Manual: 6
Maximum power: 225 HP @ 6000 RPM
Engine displacement: 2522 cc
Maximum speed: 241 km/h
0-100 km/h: 6,8 sec
</Description>
</Car>



</Cars>

I'm putting the code in the way the instructions at the top of the .xml file tells me to.

Well that way is WRONG.

If nothing works, then this is the last thing to do:
IMPORTANT: Backup the "Forward Development" folder under "Documents"

1. Uninstall the game from the "Programs and components" menu in the control panel.

2. Manually delete the folder, where the game has been installed (if it will remain after uninstallation).

3. Delete folders "Multisoft" and/or "Forward Development" from your "My Documents" folder.

4. Install the game again (using the latest distributive from our official web site) and surely run the distribution kit with administrator privileges (right click on it and choose "Run as administrator" in the context menu). During the installation process you should agree on installing all additional software, because it's necessary for correct program operation.

I'm not sure you understand. You see, when I add on a new mod, all the mods disappear, but when I delete the newest mod, the mods that I added on before work again.

What mod was the last you added? And above I said "if nothing works". That means it's the last thing to do .

I think the last one was the Hyundai Santa Fe mod, then I tried to install the Chevrolet Bel Air, that didn't work, so I deleted it, then tried the Chrysler 300C SRT8 (the working one), that did the same thing, so I deleted it. Recently I tried the Honda Civic Si, that worked, so I then tried to put the Ford Focus 3 ST, boom. Same thing again. It's becoming annoying, to be honest.

UPDATE: I notice that when I install the mods by Daeman, they work, but when I installed the Chrysler 300C SRT8, it didn't work, so I changed the .xml file to how Simulationdude said it should be, and well done Simulationdude, it didn't make a jot of a difference at all. No difference at all.