Try unplugging each rear lamp in turn. When the problem disappears you know the side you just disconnected has the fault!
Check the condition of the plug on each rear light for corrosion, check for decent earth on the faulty side at the plug (as Stani pointed out)
Check the condition of the bulb holder (Does it look corroded? Does the fault change side if you swap the bulb holder unit from one side to the other?)
Check the stop tail lamp bulbs aren't in upside down (As I think Frank hinted at)