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Well, I must have been reading your mind, because I just came in from doing exactly what you mention.
I recently installed a small DIN relay on the right fender to de-power the Radio Suppression Relay (because for some reason it would stay engaged and draw about 60ma even w/ the ignition off, killing the battery in several days or a week if the car wasn't driven). I wired the relay control coil to the ignition coil for it's switched 12v, and used the 4-tab ground strip on the fender behind the right headlights (the ones that were not working today) for the coil's ground. Coincidentally, that's the same grounding strip that the ground for the right headlights use.
It struck me that if the current for the headlights through a questionable ground raised the potential of the ground strip from Zero to high enough that the headlights were out, it was ALSO enough that the new relay would not stay closed (since the coil gets it's ground from the same strip, and needs at least 8v across the coil to stay closed), which meant that the radio suppression relay would then open, which in turn would kill the engine just like we were seeing.
I removed that 4-tab grounding strip and wire-brushed it, as well as the 6mm bolt securing it to the fender, and the contact point where the strip touches the fender. I also cleaned the ground for the battery that attaches to the top of the right frame rail, and the firewall ground and the valve cover the braided ground uses. I also for good measure removed and cleaned the connection for the injectors on the intake manifold.
I also removed and cleaned all the battery hardware, and put dabs of anti-seize (that has lots of metallic powder in it) on all the metal-to-metal connections I cleaned to inhibit future corrosion. I checked all the connections w/ an ohmmeter and got .1 ohm or less everywhere.
Then I ran the car, and put the headlights on high beams and checked the potential of each component to the battery negative terminal and got 25mv or less at the push on spade terminals on all the ground connections. The potential at the headlight bulbs themselves were on the order of 200mv, but that was due to the resistance in the wiring harness, and not the connectors.
HOPEFULLY, this will prevent a recurrence of the anomaly I had today (but of course my wife will never trust the car ever again). I wish I could have duplicated the fault before "fixing" the grounds so I could have witnessed the phenomena, as this would have increased my confidence in the "fix."
You mention the LEFT fender in your post, but I didn't see the strip there, and will look tomorrow and get that one as well, though since it was the right headlights out, and that's where I grounded the new radio suppression relay control relay, I think this one may be the more likely culprit.
I am sure there are more such ground points all over the car waiting to throw me a curve, but the ones inside the car at least should be subject to less corrosion (the one on the right fender was covered with white powder corrosion that resisted a rotary wire brush even).
Thanks-
Bob
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