The fuel pump relay is on the firewall under the hood
Reinhard - on your 1987 420 SEL, the fuel pump relay resides on the firewall. As you face the engine, you will see three 'boxes' on the firewall to the left of the brake booster. The first (about 1" square, usually red with one or two fuses under a clear plastic cap on top) is the Over Voltage Protection (OVP) relay. The next two are the climate control relay (mine has the word 'Klimat' on top) and the fuel pump relay (mine has a rev limit marked on top). Mine are black with white paint markings, the tops measuring some 1" x 4". Some time in the late 80s, the factory reversed the position of these two - on my 1990 560SEL, the fuel pump relay is on the left - your 1987 may be in the center, but there's no confusing the two based on the markings. If all fails, pull one and see if the car will start - if it does not, you have pulled the fuel pump relay.
I'm inclined to place a dab of white paint near terminals 7 & 8 on the relay socket, as you can bet the thing will fail on a rainy night in the Bronx and your flashlight batteries will be flat, so you won't be able to see a thing. Yes, I do carry a piece of (insulated) wire for this eventuality.....