Last night all lights suddenly cut out-including brake, running,turn signals, warning flashers, power windows and mirrors, wipers, dash lights and all gauges, headlights, radio and maybe one or two other things I can't think of. Former Disney in Fla. service vehicle, has Code Alarm brand remote start that always works, now doesn't respond from either remote. Mileage is 145K. Drove home 10 mi. waving my cell phone flashlight!
Would the issue be the ignition switch? A Relay? Fuse? Or the computer?
Please help. Thanx!

 

have you checked your fuses?

 

I would get a code reader on there and see if it has a module that is not communicating for some reason. To have that many systems not working is more than just fuses and relays, because these are multiple circuits in the vehicle.

Were I to troubleshoot I would start with the remote start connections, or try disabling it on the vehicle and see if that makes a difference. But this kind of problem may come down to a lot of testing circuits and following wiring diagrams to try to find what these circuits all share in common.

It could even be as simple as a ground connection somewhere has failed.

 

All fuses are good, multi-tester showed 12.95-14.22V at each fuse. So charging system is good, new Diehard couple weeks ago.

 

William is right. Find out which circuit has all those accessories in common, and then investigate on the vehicle.
Schematics are available on gmupfitter.com