OK, I think this is your answer. I found what I was looking for in the wiring diagram for the 380SL. In a completely different file, I found the following.
The white and brown wires from the impulse transmitter (aka VR sensor) go directly to the speedometer module and connect on pins 1 and 2. I don't have a spare cluster to look and I don't have a picture of the back of the cluster, but my memory is that there is one connector with 2 pins and another connector with just 1 pin, which I would take to be the VR sensor and the output to the cruise control respectively.
Assuming that they didn't change things substantially between 85 and the year of your donor cluster, I'm quite certain you'll find a two pin connector with white and brown wires.
You should have the VR sensor on your transmission, so it's as easy as running a couple of wires.
You can easily test it on the bench by using my test setup -
http://www.peachparts.com/shopforum/2883786-post9.html
If you use a hole saw like I did as a test platform, just be careful.
