The Plateback Pack Mod includes more than 30 new license plates for GTA San Andreas cars from different states and more will be added over time!
Installation:
Select the textures you like and place them in vehicle.txd, located in GTA San Andreas/models/generic/vehicle.txd.
Textures are named plateback1, plateback2 and plateback3 for LS, SF and LV respectively.
Or use Modloader.
Enjoy!