Fuel pump relay is normally behind the battery under a plastic splash cover on the W124, but a custom body, could be anywhere near that.

Are you putting gas preserver in the tank when you aren't driving the car? 5400 miles since 1990 would indicate to me long periods of storage, and without gas preserver in the tank, the fuel evaporates and gets gummy. This then clogs the fuel pump and filter, so the pump won't develop enough pressure to start the car.

If you are going to actually drive the car occasionally, you need to do so on a regular basis -- I would personally recommend a drive once a month or so, weather permitting, long enough to get the entire car up to operating temp, including rear end and wheel bearings. This is at least 20 miles, 40 would be better. I would still use gas perserver in the tank, too.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!