This archive contains backup of all GTA_SA cars except these-

1.freight
2.freiflat
3.streak
4.streakc
5.tropic
6.bf400
7.euros
8.copcarvg
9.pheonix
10.bagboxa
11.bagboxb
12.utiltr1

The backups can be installed by GGMM which is included in this 
Just Extract the ggmm archive to the Gta Sa dir and installation is done 
Also Extract the CarBackups folder to gta sa dir
Now you can apply the backups by simply selecting the car, right click on it and select Restore from Backup

Thank You