They don't have a ground connection as such; they have unswitched +12v on one wire and switching is done on the return line, by switching to ground in the front or rear (as appropriate) electronic module. It's done that way so the module can detect a failed bulb.

 

Ah...all right, then is it likely that the fault would occur in both the front and rear modules? Would I look then for a ground fault at each of these modules? And thanks for this information...the clouds are parting slowly.

 

I would think that if either module had a ground connection problem then marker lights not working would be the least of your problems.

 

Yes, I do agree; logically there would be other issues - and there are no other problems...so that leaves me back in mystery land: why do none of the 4 marker lights work?