After Market stereo
 

So I finally decided to install an after market stereo in my 84 300D, PO had removed the stock and install an after market. but the fader wheel on the center counsel was not wire properly. Hence rear speaker did not sound good.

How do I connect all the speaker wires from the after market stereo with four sets of wires to the two sets of wires for the stock stereo with the fader on the counsel functioning.

Right now just to have sound, I have the front speaker wires from the after market stereo connected to the two sets of wires. the rear speaker are getting something but not as good as the front. None of the rear speaker wires from the after market stereo are connected.

You will need to connect two of the the outputs from the head unit directly to the fader. The fader should then take those two and output four.

The rear outputs on the head unit are left disconnected.

Typically folks bypass the fader as they go bad, get dirty etc.

Most new stereos have 4 amps. The fader is meant for a 2 amp system.

In an old 123/124/126 with the fader you have to bypass it completely and get all 8 speaker wires isolated and connected to the head unit's outputs.

It's not too hard to follow the wire pack from the radio to the fader to the speakers.
Or just run all new wires.