Are you looking for the install the CD unit in the dash area or in the trunk?
When I installed my changer in the trunk of my 90 300CE I picked up the constant power and switched power from the power antenna plug in the trunk. One pin had constant power and another had swithced power that worked with the radio. The switched power worked regardless of the antenna switch on the dash and the ignition key. In other words...if I could turn the radio on, the CD changer got powered up....when the radio shuts off so does the changer. Very simple.
This was the quickest and cleanest way I found to isntall my CD changer. Only wire(s) I ran to behind the radio was for the FM Modulator and Wired remote.
I'm not sure if this is what you want to do....hope this helps.
2003 SLK230 (Sport - 6spd)
1995 Ford Thunderbird