Originally Posted by cdgatti
By just hooking up to the harness, I would think that would be sending head unit amplified signal to the amp. Not a good thing.
If the harness was really intended for a hi-fi HU, you would think that it would have RCA pigtails coming off it since the manufacturer should have known that the OEM amp needs a preamp signal. Otherwise, you might be right in saying that the harness might have been intended for a base audio system. Still, the OEM connector behind the HU is the probably the same no matter what HU is installed and I'm thinking because of that, the speaker level outputs for the base HU and the balanced diff outputs from the hi-fi HU would probably not occupy the same pins. Therefore, if you connected a base audio harness to the hi-fi HU, those hi-fi amp input in the OEM harness are probably connected to nothing right now.
Of course, I've never looked behind a hi-fi HU or base HU so this is all just guessing at this point.