Can you show both pictures in-game of those vehicles, and the script you use?
These are the two vehicles already in the default main.scm for which the opcode doesn't work (at least here):
09E2: $5186 = parked_car_generator_w_numberplate #BANSHEE colors 79 0 force_spawn 1 alarm 0 door_lock 0 min_delay 0 max_delay 10000 plate "__DBP___" at -2273.2 -130.8 33.9 angle 270.4
014C: set_parked_car_generator $5186 cars_to_generate_to 0
09E2: $5185 = parked_car_generator_w_numberplate #BULLET colors 24 40 force_spawn 1 alarm 0 door_lock 0 min_delay 0 max_delay 10000 plate "_CHUNKY_" at -2354.6 983.0 49.3 angle 189.4
014C: set_parked_car_generator $5185 cars_to_generate_to 0
There's really no need to post screenshots and go into detail, it is what it is. The opcode simply fails for these two (and some other vehicles I try adding, such as the Stafford). Apparently, once they fail to generate properly after you start a new game, they're stuck as regular random plates for the rest of the playthrough.
I just want to understand what's going on.