'89 4CH Non-Prem Sound, No Amp or Fader Head Unit Wiring Question

[AWDBOB]

Hey R3V (or probably just bradnic )-

I have wired many an E30 head unit, but the car I have is unique and don't quite know how to go about wiring it up.

I have an '89 car that came with with a 4ch (5908), with no premium sound, no fader, and no amp. The 5908 is inop so I picked up the Continental TR7412UB-OR for modern functionality.

My question lies in the rear speaker ground wiring, since the 4CH used the adapter with a common ground for the rear speakers.

After the BMW adapter harness the Blue/Blk is for the right rear speaker +, Ylw/Blk is for the left rear +.

Should I just pull these ground caps off and run a wire from the yellow ground junction to the LR and blue ground junction to the RR? Or do I not use the adapter harness and figure out what is what behind the harness?

Pic of the ground junctions:

Untitled by Bobbie Morrone, on Flickr

[AWDBOB]

I just grounded the rear speakers to their respective common ground and it all worked out great, if someone else stumbles on this in the future.

Untitled by Bobbie Morrone, on Flickr

[earthwormjim]

I left the speaker channel grounds disconnected on my Continental TR7412UB-OR, since my car had common grounding for all speakers.

It's a wagon, and I didn't feel like digging around under the carpet trying to find the ground connections, since I couldn't find a straight answer on where the common links are.
Also had an unusual configuration which I don't think was ever offered in the US, so documentation was scarce. It had premium speakers and tweeters, and the fader dial, but no amplifier. Pretty sure that's how it was factory.
Some sort of partial premium option.

So my connections are just speaker "+" in the radio, out to each respective speaker, with the speaker's ground still tied to chassis ground (common ground).
The radios "-" connection for each channel is just left floating. I removed the fader.

It's been working fine for nearly 2 years now.