Radio Gamma CC to Bose = mysterious?!

Good evening.

I have a V8 here that has Bose. However, the rear speakers have been replaced for accessory speakers.
A Gamma CC (without Bose outputs) should now be installed in this car.
Not so heavy in itself, however, all cables are cut off and the original plugs no longer available, as so far an accessory radio was also installed.
So I started to find the original belonging with the Gamma CC and Bose occupancy plans.

The following difference arises from the Bose to the Gamma:
The Bose has 4 pre-amplifier outputs and the Gamma CC has only 2 (for the rear speakers).

The thought was now:
The rear accessory speakers to run through the reinforced front channels and the front speakers (bose) to run through the rear pre-amplifier outputs.
The rear ones are now running over the reinforced exits.

The idea was good, but the following problem arises:
No matter which speaker cable I hold to a pre-amplifier pin, only music comes from the front left speaker.
The right front speaker cannot be persuaded to play music.

How is that possible? Even if I hold the cable for the right front speaker on one of the pins, music comes from the left (front) speaker.

Maybe. Any idea?