I have seen a couple of things. First check all the lamps, sometimes someone replaces a bulb with a single base bulb and it shorts out the turn brake and parking lights, no damage but a lot of confusion. When doing this check for corrosion on the sockets as well as the bulbs. Smear a light coating of grease on each bulb to reduce this.
As you remove each bulb, check to see if the symptoms change.
As an aside I have also dual filament bulbs configured wrong. Dim when brakes bright when parking. It has always been a tin plated (silver) never a brass bulb.