I think something else is wrong. There is obviously a bad circuit. You have replaced the starter portion of the circuit (twice without repair), you need to evaluate the problem; which is real simple if the problem exists when you are checking.
You need three things to happen for the starter to work. The engine block needs to remain a ground, the large cable from the battery must deliver full battery voltage to the top post of the starter, and the 50 terminal from the ignition switch must bring battery voltage to the small solenoid activating wire.
All three needs are easily verifiable with a volt meter using the voltage drop test. Test the ground first. To do the test one would place one lead of the voltmeter on the ground battery terminal (not the cable but the terminal inself - touch the center not the connector) and place the other lead on the body of the starter. Try to crank the car while viwing the volt meter. Because both leads are on ground there should be no voltage. An acceptable voltage (voltage drop) would be .1v maybe .2 (in reality). If the connection is the problem the drop may be as high as full battery voltage. By testing from the center pole to the starter body the whole ground circuit is tested. If it flunks, each component can be tested till the problem is centered using similar technique. If the test showed high voltage drop I would next do the test with the lead on the center of the battery but the other lead on the cable connection attached. If good move down the line. MB batteries ground to the frame so there must be another wire carrying the ground to the block (usually from the trans to the frame). Follow each segment and till the point of the voltage drop is found - the volt meter won't lie.
Do similar testing to the positive cable which has fewer connections as it does go straight from the battery to the starter solenoid. There should only be voltage drops through resistance. In theory there should be no resistance to wiring.
Bosch Master, ASE Master, L1
33 years MB technician