Strange turn signal problem

Unplug ALL the lamps front and rear (turn/park/stop) then check the housing to ground, if a housing has a bad ground it will find ground the the positive circuit to another bulb.
I'm not an electrician so I can't explain it.

That’s right. These turn signals and brake lights will have you chasing your tail because of that ****! When you see dimmer than you expect lights that’s usually a good sign it’s getting ground through a bulb turned resistor.