Let's start with the car's installation:
There are 2 ways: I.Simple
II.Advanced
I.Simple:
STEP 1
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Download this program : SAMI
After installing it, launch it and set the Game Directory and set the location of the mod (must be .ZIP or .RAR)
STEP 2
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Press next and again next and now select what car should the program replace(Car to Car is correct, but Car with plane would crash the game)
II.Advanced: In the advanced section you can edit everything but harder.
You have to download this program.
STEP 1
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Open IMG Tool 2.0 and click FILE>OPEN, and open "gta3.img" located in "/GTA san andreas/models"
STEP 2
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Click COMMAND>REPLACE, and select the files from the mod and the rebuild archive from "COMMAND>Rebuild".
You are done...Enjoy!