Wiring 2 channel AMP to 4 channel car

[onesecond]

I'm looking to go back to an OEM radio and picked up a CM5905. Sadly I didn't realize that the older cars used the slider and plug to convert to 4 channel while my 318is came with the 4 channel setup (non-premium sound). From my understanding if I wire into the fader plug I should be good but I was wondering if I would encounter grounding issues? The picture below is the current mess I have in the car. The owner for some reason installed (not connected to anything) a fader, so all I need is the plug 61101388934?

[bradnic]

Reach out to me bradnic@oemhifi.com for assistance with your radio and harness. The correct radio for your car is the CM5908