I'm sorry this is so vague. I know there's a remote turn-on issue with the factory amp if a different head unit is installed. I'm very blind in this area and would probably start at the12volt.com for a wiring diagram. You mentioned a previous owner. I'd be willing to bet they couldn't get the amp to shut down after putting the stock radio back in, hence no fuse.

I doubt you'll need to do a bunch of wire tracing or anything. Pretty sure it's something easy.