What a coinc-i-dinc!
I just posed the sme question to Gernold at SLTech with my car. Here's what he said:
"The Fuse Box is original and it was used for the original Radio wiring, the Wire coming from the Harness should be Red/Green and the Wire from the Fuse Box going to the inside of the car was Black, this set up was wired to the Ignition Lock position I for Radio use without turning the Ignition on all the way, I believe the Wiring Harness I sold you had the Radio set up on it, earlier 230 SL did not have this set up,
Hope this Helps and Good Luck with your SL,
Gernold"