When I open the game and select a car to drive and press "Start" my game crashes and says "could not open source file"
Any suggestions?

You can try this to solve it:
If your PC has both – discrete and integrated video cards, then maybe the game uses the integrated one for some reason. Try forcing the game to the dedicated one – right click on the game shortcut and select Run with GPU <your dedicated video card>. Or if you don’t have this option in the context menu, you can follow these advices – www.techadvisor.com/how-to/pc-components/how-set-default-graphics-card-3612668/

If not, try these steps:

1. Install DirectX and Visual C++ components from this folder manually, go to this folder -
"...Steam\steamapps\common\City Car Driving\_CommonRedist\" and install all the components from there.

In some cases this folder can be located in a different place - "... Steam\steamapps\common\Steamworks Shared".

2. Update your video card driver to the latest version from your video card manufacturer's website.

3. Update your sound card driver to the latest version from your sound card manufacturer's website.

4. If you're using a racing wheel (or any other controller), update its driver to the latest version from your controller manufacturer's website.

5. Turn off any antivirus or firewall software.

6. Check the game cache integrity in Steam client.

7. Turn off the Steam cloud profile synchronization for City Car Driving – open your Steam client, click on City Car Driving with right mouse button and choose “Properties” from the context menu. Then open the tab “Updates” and there turn off the option “Enable Steam Cloud synchronization for City Car Driving”.

8. Delete the folder "Forward Development" from your "My Documents" folder.

Then run the game again.

I think it's because of some mods that are not working properly, is there any way to delete all mods to the game?

You can either delete the codes from the user_cars folder (C:\Program Files (x86)\Steam\steamapps\common\City Car Driving\data\config\user_cars)

Or delete the mod files by searching for the code of each car in your CCD folder, and deleting the files: