Check out this place:
San Andreas FilesThe second most downloaded file is the [S]an [A]ndreas [M]od [i]nstaller, or SAMI. This is a great tool for easily installing mods, which includes: skins, vehicles, models, clothes, textures, etc.
Also, you could try
GTA Garage Mod Manager, but you'll need to use GGM files with this one. Even if you don't use it to change your cars, it's always a nice program to view them.
However; your best bet would be to use the
IMG Tool. This is going to be the most reliable, yet not as user friendly. You're not having to deal with special files, such as with GGMM, or going through the install process of SAMI. It can be a little confusing, but there are people who are willing to help.
Good luck. And I hope this info hasn't already been posted.