are the lights staying on constant or flashing (as if they were indicating)?
when the fault appears, take one indicator bulb out at a time to see if the fault moves / changes
Are the lights on fully or just "half on"?
I would check:
correct bulbs installed, should be 12v/21w... make sure no one has forced in a 12v 21/5w stop and tail bulb
any trailer sockets (that are there or were there.... cut wires?) look for corrosion
any cheap alarm wired into to the indicators thats faulty
no power leakage from dash, e.g. 12v coming in on ignition and jumping across a corroded circuit board onto the indiciator circuit
water in fusebox / relay box. sometimes its easier when the fault occurs is start pulling the fuses and relays out one by one to see what difference it makes. make sure to plug them back in one by one ;-)