If you have the engine manual, look at job 073-165. It shows a lot of wire diagrams involving the fuel pump and relay. Basically, there are a few possibilities depending on how creative you want to be. You should also look at the ETM.

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino