I recently installed a rebuilt starter in my 1972 250C project car which I purchased on e-bay, and has been apart since 1992. Not a problem, except that I have an extra wire - looks like 2.5 mm, and gives 12 volts with the ignition on. It has a good-sized terminal on it, bigger than the ignition switch lead, but smaller than the battery connection. It is the same length as the other starter leads. I cannot find anything in the wiring diagrams, nor is there much in that area that it could connect to.
This car has definitely had some rewiring, so it may not be stock.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe