my question is, if the park light bulb is completing the circuit from the socket hot wire to the light body, when the test light lights, why doesn't the park bulb light? if that ground spot in the headlight bucket is an actual ground, then why does the light show a difference in potential [a complete circuit] from that ground to what should be another ground a few inches away?
does the park light unit have lock washers on it's mount bolts? does the grill have lock washers on it's mount bolts? are all bolts tightened enough to make the lock washers cut into the multiple layers of paint? have you tried the test light probe to the grill near the park light? the only thing I can see is that you've insulated the light from the grill somehow
I'd take the light assemblies off the grill and try the lights with wires hooked right to the battery, which would tell you if the bulbs are good, and if there's a short from the socket base to one or both of the wires internally - then when remounting make sure there's good metal to metal contact at the mount points
Bill
