I think a unit will only have it's lights on if it has a driver...
I was in the same situation you were a while ago, and I tried damn near everything to get a jeep to keep it's lights on.
This was for a cutscene, so in the end I created some M2 machineguns and suspended them in the air out of camera shot
Lit up the area very well, but it wouldn't have worked if it hadn't have been a cutscene (floating Machineguns kind of kill the mood ;D)