Here is an explanation solution, regarding the fuel pump relay burn out problem on the 200 series. I (had) a 1978, 265 GL wagon and still have well preserved 1981, 260 GLE. Both of these cars produced the relay burn out symptoms as you describe. However, I seem to have eliminated the problem by soldering the wire and connector to correct resistance heat. The problem seems to be poor connectivity at the terminial end of the wire. After the cars were at least five years old, the problem of heat and a burned out relay began. Time is worth noting since there is no other known variable. With time and the various effects of aging in mind, it is possible that the factory crimped wire and connector did not remain as a good conductor of electricity. Since I soldered the wires and connectors and then reconnected the new relay, there has been no heat and no relay failure. Previous to this repair, the relay would last about 12,000 miles.
Repair: Unplug the relay and cut back the wire insulator just slightly. Prepare the surface areas and solder a bridge between your wires and connectors. You do not have to put on new crimped connectors. Re-insulate the exposed areas with a liquid insulator. Re-connect the relay.