Here's my drawing lol.
So imagine stepping on the brakes. Power comes down the red wire on the left and is spliced right there so it also goes over to the right. On the right it should be on the B
but it's accidentally on the T
On the left the B
filament lights up, and on the right the T
filament lights up.
Here's where I'm not 100% sure. On the right side the power coming in the T
can back-track through the bulb and also light up the B
and if it did the ground path for right's B
would be through left's T
. That would explain all 4 lighting up, but the 2nd "pair" should be dimmer. And even without crossing the wires, backtracking is still possible so my first guess of crossed wires isn't making much sense...
Also I'm pretty sure electricity wouldn't back-track like that, the ground for right's T
is the G and it's right there. The path to back-track has high resistance because of 2 filaments, and if the local Ground is good there's no reason why all current wouldn't ground right there.
Wait a second, maybe lacking a ground is the problem. Take ground away and now there's a reason for electricity to follow a strange route.
Even if the wires aren't crossed up, if the right bulb's ground wire is broken or bad, now the ground path for power going to B
on the right is to back-track through the bulb, through the T
filament on the right, which will also flow through the T
filament on the left lighting it up, and ultimately through the left bulb's ground.
Check your ground wire on both of your rear lights and make sure one of them isn't bad.
I'm just making stuff up over here though, none of this theory applies if the circuits don't share a common ground. I wish I had a 68 wiring diagram.