I don't know about Mercedes but Ford GPs can be tested in place. Pull the lead going to the GP and put an Ohm meter from from the top of the GP to ground (head). 1 Ohm or thereabouts is good, open cct is bad.

When the engine is hot the GPs are not really necessary so it sounds like something else. Do you have any fuel leaks, places where air could be getting into the system when it is at rest.