C3500 electrical issues
 

I have an 89 Chevy 3500 with the 454. It's relatively new to me. The battery gauge started pinging back and forth and the electrical stuff was cutting out on the way home. I figured the alternator was going out. Then suddenly it all shut down and wouldn't even attempt to turn. I even tried a jump pack and nothing. I put in a new alternator and fully charged the battery which is only months old. Still nothing. Then I realized that none of the power accessories work when the ignition is turned on. No fuel pump, no radio, nothing. I have lights but that is it. There had been some aftermarket stuff put in like a radio and such by the previous owner. I don't mind swapping parts as they are relatively cheap but I'm really not sure where to start. Any ideas?? Thanks!!