Cruise Control amplifier reference resistor question
 

I have a line on a good cruise amp which should work in my '85 380SE, and probably in my '86 560SL as well. The problem is that it's a newer style and requires a 'reference resistor' to work.
I checked the on-line parts sources, including here of course, and they all say that one of these is required in the description of the rebuilt amp, but don't appear to sell them as separate items. Do I need to go to the dealer for one of these? $urely not.
And what is the function of this thing anyway? I've read that the plug contains some resistors on a circuit board. Does anyone have a second hand one that I could copy, if that's all they are?