Jack,
I am not really familiar with your SL and I haven't done this to mine yet, but personally I think putting in a cut-off is a good idea. If finding the fuel pump power feed is a problem, think about this alternative.

Again, you may need input from a MB guy, but I used an RPM signal on my 1974 3.0 CSi BMW. The computer requires an rpm signal within 5 seconds of ignition "ON" or continuous while the engine is running. If no signal, it switches off the fuel pump. This is a safety percaution in case of a wreck and the engine quits, but ignition is still on. In this car, it was a heck of a lot easier to find the rpm feed than going back under the rear of the car looking for the fuel pump feed. It worked perfectly and never gave me a problem.

Just a thought.

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Coast Aero Support Services, LLC
'92 MB 500 SL
'01 BMW 525i Sport
'98 Harley Davidson RoadKing
'74 BMW 3.0 CSi - Gone to a good home