When we find a clunker, the quest for the bad bulbs usually leads to some really creative expletives at my house. Thank goodness Santa brought a gift early this year — the chaps over at EDN magazine ...

We have all had difficulty testing diodes in-circuit. Most DMM’s have a diode Vf function that measures forward drop, but what is the normal voltage drop?

This is the ground circuit. Another color will lead only to the taillight, stop bulbs and side-marker light. This is the positive side of the parking light circuit.