Ok, so I'm putting my old Clarion deck in my buddy's 1990 Doral Concept....

There are two wiring harnesses in his boat's dash. One is for power, the other is for speakers. The speaker harness has 5 conductors. 4 of them are labelled: RF+, LF+, RR+, LR+

The last wire has no label. Now, I remember old car stereos (installed in a car) used the car ground as a common negative terminal. Newer decks have obviously gone away from that with discrete positive and negative leads. But how do you hook this up in a boat? Especially as a boat's, ummmmh, chassis..... is not grounded. I hooked all the positives up to the new deck positives, and then one of the decks speaker negative leads to the fifth wire in the boat's harness, and I do get sound from all the speakers, but the back ones are very quiet, even with the fader adjusted evenly.

At a bit of a loss here....anyone know? Short of running new speaker wire....

I don't really know much about the old style wiring, but I would think that you would have to tie a common ground lead to all of the negative speaker terminals. That may not be it. Personally if I were doing it, I would just run the modern speaker wires through the boat. A lot easier than trying to troubleshoot a system you don't know much about. Your boat's chassis is grounded, normally to the engine block or motor mount.

That is an option, it's just an older boat and don't want to spend too much time on it.

I don't think the boat is grounded though, and fibreglass is not too conductive.

Yep, use the old wire as a fish and run new speaker wire to each speaker.

Yeah they just ran a ground wire in a loop to all of the speakers. You are going to want to pull new wire.