My apologies for asking a question that has been asked before on many different ways but i wanted to describe my symptoms to see if anyone has had this exact sort of thing happen. My son bought a 2001 Monte with 160000miles. Through an online auction. No mechanical declarations. We picked up the car and the first thing noticed was dashboard lights brake, air bag and traction control. Car wouldn't start initially but my son said depressing the brake and starting worked. Driving it home there were no signal lights but the hazards worked. Decided to drive to a parts store to pick up some bulbs but also to search the web... we were checking tonsee if fobs worked for doors and trying the keys that we got. The oldest key which I believe was original factory key wouldn't turn crank probably because it was worn. But now when we try to stsrt it we get a flashing security/battery light and after attempting to crank no turn over and the light goes to solid battery light instead of the altenating security/battery light. We tried disconnecting the battery for 40cmin and tried to do the 30 min reprogram. Ther e is a click from the relay inside fuse panel the interior display goes out when we try to crank. And there is a whirring noise from what seem like right around the instiment panel. So obviously bad luck with the purchase but love the car so would like to fix. When we do the crank on and leave it in on position. We have a battery symbol not a flashing security symbol. Do we still have to do the 30 min reprogram. Also what's up with the depressing the brake to stsrt it. Brake is stiff now. So I've read bcm. I've read ignition switch. I've read bad battery maybe a starter. So we will tske it in for diagnosis but don't want to get gouged. It would be nice to know thoughts ao we know what we may be dealing with. Hopefully this all makes sense lol. One last thing when we initially were trying to do the reprogram the head lights and tail lights stayed on for s bit and then went out when in the on position. Should these stay on or off. By unlocking and locking doors several times with fob or trying the one key that didn't turn in ignition. Could that have cased the car to go into anti theft. There is no ket symbol.

 

6th gens always have issues. Is the Stock Radio in the Car?
I don't have one and won't get one because of some of these little issues. But I think I would find a salvage yard to get a different BCM for it and just see if that helps.
Any GM that I have had when that goes out the issues are similar to what y9ou describe.
There are a few experts on here that may have some great answers .