Chances are there rear brake light switch is adjusted so it is on all the time and the brake light filament on the right side bulb is burned out.
If the right side comes on when you apply the brakes and the left is on all the time, some electrical work was done wrongly. W/B is brake light hot, B/W is ground and Gray is tail light hot.
I changed both bulbs today and the right side is working fine on and off. The rear brake lever needs persuasion to return to the off position, but surely a switch problem would effect both sides?