Replace these line in GTA San Andreas/data/maps/veh_mods

1001, spl_b_bab_m, infernus, 70, 0
1006, rf_b_sc_r, infernus, 70, 0
1011, bnt_b_sc_p_m, infernus, 70, 0

If you're lazy to put these lines one-by-one, 
just replace the veh_mods with the given ones.