I've searched on the SA:MP forums and found multiple topics.
23 pages about the problem 1) Go to your My Documents/GTA SA User Files Folder.
2) Delete the file named gta_sa.set. This file contains your game configurations, the game will recreate this file when you start it next time with the default config.
3) Start up SA-MP, dont change any config settings. Go to a car mod shop.
4) If you manage to mod a car, please post back here, if you dont, go to step 5:
5) Start tinkering around with other settings (changing resolution, Visual effects, AA, etc.)
Cheers,
-xcasio

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- Eleanor Roosevelt