Brake lights swapping when signaling - 61 Galaxie

[dmsfrr]

Originally Posted by NoahVail

We just pulled the leads and cleaned them. No change unfortunately.
We can still reactivate & deactivate each brake light from the turn signal lever.

Is there a dedicated ground wire from each tail-light socket to the body, or do the tail-light housings ground to the body thru their mounting screws?

If it's not the ground connections there may be a poor connection in the wiring harness or turn signal switch, as you suspected. (since it sat out in the open with the top off for a while)