6th gear doesn't work on any of the modded cars I have and I've tried everything. The only gears I see are 1-2-3-4-5-R. I've also tried the AT only bug fix and it still doesn't work. Anyone have a clue on why this is happening? (I have the g29 + shifter steering wheel and in other games the 6th gear works so i don't think it's a shifter problem)

Hello,

Modded cars all have the same gearbox icon, which only shows 5 gears. This does not actually limit them to 5 gears.
You can still shift into 6th, and while the icon will remain in the 5th gear position, you will be in 6th.

You will notice the RPM drops down when you change from 5th to 6th, which confirms the gear change.

Did you actually fix the issue TJ told you in the end? Interestingly I found out if you want the icon with it, I don't know if it works but you can try it if you wish if you want that go in to the CCD folder:

export/gui/Common/inagesets\transmission

And for what ever mod you want to use it give it the name of the mod just copy and paste the folder with the files inside it and put the mod name. I think some of these have 6th gear icon, But I don't know if it will work I haven't tried it myself maybe you can if you wish to do so, If it doesn't work just delete the folder you made it shouldn't affect anything if done correctly of course.