2003 Isuzu Rodeo Question Brake lights

Hi there,
When my lights are on my right rear light is dimmer than the left one and when I press on the brakes the r/r light doesn't light up at all. Also, when the lights are off and the brake is pressed the r/r brake light is dimmer than the left on. I tow a boat with this vehicle and in the past I have had some problems with the wiring harness shorting, but it never effected my vehicles lights.

What do you think?