You just need the wiring diagram for the "radio" for a 1989 model W126. They will all be the same. The FSM has it, or you can find it on the Startek site or Google should provide the answer. The factory wiring isn't anything special, whoever says they can't get the info on it is either lazy, ignorant, or lying.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)