electrical problem

[bucketobolts]

I'd look at the left rear tail light again. I'd double check the bulb and the socket.

I had a similar problem once on Trans Am. All lights appeared to work correctly, except when ever you would apply the brake or right turn signal the headlights would flip up and down. Had a fault "new" bulb with the elements crossed inside. So it would back feed the park lamp circuit when ever the bright side was powered.