Dashboard tail light driver side is on.

When lights are turned on and brake or turn signal operation alters the brightness or causes other lights to operate the problem is nearly always a bad ground ,(or ground at the bulb holder), or an incorrect bulb fitted (for instance a single filament bulb fitted in the socket where a double filament bulb should be). Try removing the rear lights bulb holders look for corrosion due to perhaps water leaking in through a lamp or seal, also try pushing on and off the electrical connections at the lights a few times to see if it improves conductivity.