All you need is a switch to control the relay. The switch, and its wiring will carry verry little current, because the relay switches the heavy load ( that is what they are for).
Lots of Ford IDO owners have done this as they have 6 volt GP in a 12 Volt vehicle. I its how they get the fast start). The Ford controller is designed to cycle them, because after about 12 to 15 seconds of 12 volts, all the GP are toast.

I do not see that having them come on in the MB will harm anything.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags