So I got an '99 EB that has had the HU changed to aftermarket. My issue is the rear sub doesn't work. I'm getting power to it but either no signal or no turn-on signal. On the HU they only have one harness plugged in. Does the Mach system in the '99 have 3 plugs (I see 2 plugs that have nothing to them) My front and rear door speakers work great with no issues. The wiring diagrams I've found ('98 mach system) show only one blue wire from the radio out to the front amp and cd player. Out from the front amp to the rear amp. However my front amp obviously works. I'm thinking that one of the 2 plugs that are empty behind the radio are to the cd changer and the sub amp. I've bought a shallow mount pioneer 8inch sub and if I have to I will change the amp to an aftermarket but at the moment I'm trying to keep it a little on the stock side.

 

Okay through some extensive research and buying wrong adapters it looks like the Metra 70-5517 is the adapter I need. It only has one rca jack and my harness at the old radio harness and going back to the amp only has two wires for signal. One of the plugs that wasn't plugged in is to the back amp. All wiring diagrams I can find don't show a seperate connection for the cd player and amp. Also I think the '99 is a change point in the wiring of the Mach radio system. Even the Metra site is recommending the 5517 (one rca jack) or 5519 (two rca jacks). I'll update once I get the harness and see if it installs correctly. After all of this I hope the amp works lol.