That GP system is screwy and I can't figure why MB ever used it. The GP are actually 1.2 Volts as if they were designed for a 6 volt system. The wires between the GP are resistance wires to drop 6 volts or so. They are supposed to get hot, so don't replace them with copper wire.

Does you GP indicator glow orange hot? That is a good test of the GP system. The GP are not turned off when the starter is engaged. When you pull the starter handle to the start position, the dash indicator is shunted out (the glow disappears) to give a slightly higher voltage to the GP which remain energized when the starter is cranking the engine.

How fast does the starter crank the engine? A few fast cranks are better than a lot of slow cranks. It could be you starter or battery are getting weak. Or it could be a poor connection in the cables (either end) from and to the battery and starter or ground.
See if you can feel a hot spot in these cable connections or measure the voltage drop across the connections, it should be almost zero.

The next test for hard starting would be a compression test.

P E H