What you do first is download the IMGTool here http://www.thegtaplace.com/downloads/download.php?id=552
And download a car here http://www.thegtaplace.com/downloads/category.php?id=32
Extract imgtool.exe into your desktop. Then, backup the car(s)/vehicle(s) file(s) in a New Folder.
Open up IMGTool, and click on; C:Program Files/Rockstar Games/GTA San Andreas/Models/gta3/
Open gta3 with IMGTool and find the search for the file what the mod file says. For example, sunrise.dff and .txd. If those are what the modded files are called, just click on search in the IMGTool and type in sunrise/sunrise.dff. And then, click on replace, and then find the New Folder on where you backup your files. Replace them from .dff to .dff and from .txd and .txd.
Woot Woot! You have it installed! YaY!
Click this link for more info on how to install mods such as cars. http://www.thegtaplace.com/forums/index.php?showtopic=5959
Heres another link for installing cars. http://www.youtube.com/watch?v=5DMPVv09JeA
Have fun with the car mods!