An electrical light mystery. 78 F100

[JBMCQ]

Hoping someone has had this occur so as to point me in the correct direction to fix it.
I have a background in electric troubleshooting and repair.
Have not as yet pulled out the multimeter to test anything.
I am in the swim pool industry, working on everything from motors to CPU based controls using relays to activate the various components.
I am having a hard time visualizing what could cause this.
1978 Ford F100 2wd
The Problem: Right rear brake light / during daylight hours both
brake lights function, although it seem the right rear is somewhat
dimmer.
When I turn on the headlights/parking lights and apply the brakes
the passenger side (right rear) light goes out completely.
Foot off brake light back on / Have not tested to see which filament
in the bulb is lit. Break light bulb has 2 filiments one for tail light/one
for turn, brake.
Suspect it is a short in the harness wishing its not. So as I said
hoping for someone who had this occur to shed some light on the
subject, (pun intended) or someone to help me with procedural steps
to test + determine causes.
Thanks in advance for your help,
Jim M

 

[bronc76]

The harness for the rear lights runs along driver side frame and the plug that connects the harness to the lights is just behind the fuel tank on frame (at least that is where mine is). You could probably check right there with the multimemter to see if its in the harness or tail lights. Have you checked grounds?

 

[JBMCQ]

Hey,
Thanks for the input , exactly what I was hoping for when posted. Bad ground makes sense.
Will check and post results.
Thanks for the replies.
Jim M
If you ever have a pool question just let me know , When asked how long Iv'e been working in that buisiness
the standard answer is too DAMN long ! HA. Not a lot I havn't seen.