I have found that single cell batteries (1.5v)never gave enough voltage to really move the speakers, I always go with a 9-volt.

On the a similar topic, why did Porsche decide to run a single wire pair to the rear and then put the rear speakers in parallel on my '80 911SC?
The carpet is glued down and I am not sure how I want to run the additional wires for true stereo (not to mention my HU can't be bridged).

Porbably because the Germans were ahead of their time and knew that the human ear "canot" hear in stereo from behind when the sound is less then 10 feet apart. (try it, it's vertually impossible to tell the diference stereo vs mono when listening from behind)

Ok, so if the speaker wires are one color all the way back to the amp...but both leads are black then I would look closely at the wires. If they are standard ZIP cord type[lamp wire]they will be joined together in the center. One of the wires will have a smooth insulation and the other will have ribs [no jokes] or a seam.

Not that I care either way for the rears - stereo or mono - but I have one paired wire and two outputs. They cannot be tied together or bridged and I have been unable to find a suitable matching transformer to do the job of mixing the left and right rears into one signal (double primary/single secondary, 1:1 that passes full audio spectrum)