Best not to install vehicle mods with SAMI.
Relating to your problem, I can only think of 3 explanations:
1. You deleted the backup folder.
2. SAMI did not create backups at all, probably a Windows 7 compatibility problem. (this is most likely, since SAMI wasn't made for Windows 7) (SAMI didn't run great on XP ether).
3. SAMI did create backups but can't use them, for the same reason.
How to revert vehicles back to normal:
1st option, reinstall GTA SA.
2nd option, look in the backup folder and install the backed up / original files just like you would install a mod (but without SAMI, if that wasn't obvious ).
How to avoid this in the future:
Don't install mods