I just bought a Pioneer CD Tuner and wanted to fit it now in my Volvo 850 1994. The wiring diagram makes sense to me so far, the only thing that i dont understand, why do i have (on the original connector plug from my Volvo) 2 different black cable? And do i have to connect them both with the single black cable on my new Car Stereo? Black is ground but why 2 black cable? And how do i properly connect the 2 additional loudspeaker in the dash? On my new Car Stereo there are cable for 4 loudspeaker but i have a total of 6 loudspeaker so how can i connect the 2 in the dash??
I did the same for my S70T5 , Pioneer double DIN unit but you need to look to the wiring for the speakers as the fronts are in ( series or parallel) rears opposite ? , don't recall which way. So you need to work that out , I ran new JBLs that fitted the hole up back , no rear door speakers and the installer mucked up the fronts for me needing more work.
As for centre speaker , the only way I can see is to use an Alpine head unit that has the capability , usually a DVD player type or the old school parametric controller type you can get on eBay but not cheap.
I suppose it is possible if you have good installer...
Black wires I don't know.