Dug up the book (Chiltons) I have on this vintage car and it indeed shows a starter relay. Should be in the ECU box in the passenger side kick panel, but I don't know which one. It's not the top one on the rear bank, nor one of the top two on the front bank (those are the EFI relays). You should get voltage on pin 85 of the correct relay when the key is in start position. Jumper pins 87 and 30 and the starter should engage.
Now the fun part. I have no idea if that relay is actually used in you car!
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!