Originally Posted by BobsM3Coupe
I'll bet the local shop was just going to let the engine idle while coding the lights to the car. Does that sound safe?
No. Depending on specifics, most modules need to stay within a range of 13.5-14.5V and no loads on the sytem during flashing. Simply idling the vehicle doesn't guarantee voltage will stay within that range, If that was the case - I wouldn't of had to purchase a $1500 power supply. For me, it's cheap insurance against potential issues when working on your pride and joy.
He might get away with it once or even a handful of times - but Murphy's Law may apply to your case. I wouldn't chance it.