I assume that you can jump start the car. If so, the starter is likely working. You may have a current leak when the key is off. For older cars, the current leak should be 100ma or less (it may be around 60ma for the 300D). If you find a high current leak (using an amp meter), remove the fuse one by one to see where is the problem. The alternator may not have fuse and you have to disconnect the wires (be careful not to short the hot wire to ground)from the alternator. Be aware of the fact that a defective glow plug relay can drain your battery quickly.
I do not recall any relay near the resovoir but a blower speed resistor box, a terminal block, and an electrical coolent pump.
It is still possibly that you have a defective starter. You have not run enough tests to prove it.