I would start by holding the battery boost switch down which is on the dash near the headlight switch. I would do this with the engine running and maybe the 12V lights will slowly come back on... may take a minute. Next with the boost switch held down, operate the the salesman/batter cutoff switch one time as explained next.
The 12V battery cutoff switch by the door should be 1"X 2" and labeled "Battery Cutoff" but it's hard to accidentally shut it off... takes two steps to make it operate. Did the lights come on? If not thentThe solenoid this switch activates is not working and on my 04 Dyunasty it's in the back/engine run box. It has large red wires going to it and while you operate the switch he should be able to hear it clicking. If he does not have 12V on each side of these large wires, connecting them together should solve your problem.