Thank you for your help. The problem is now fixed by cleaning the fittings around the bulb. Problem was on a fourtrak!

I agree it sounds like an earthing problem caused by corrosion in the bulb hoder. Take it out and clean it, speray with WD and then try it. If the bulb is blakened then replace it as this indctes water intrusion and shorting. If this is the case then check the seals around the lens. You mat find algae covering the bottom of the lens itself. If that fails then check all the connectors at the rear of the vehocle for corrosion and the wires for chaffing.

It would help if you said what vehicle it is. I have assumed a Sportrak which have normal bulb holders and no printed circuit.

OLDMINMAN

Has a bulb been replaced? If the bulb has 3 connectors on its base it is possible to fit it with the base connector wrongly positioned. Wrong position will cause an earthing fault.

Otherwise check all earth connections. A 12v tester would rapidly locate the fault.

Does sound like an earth falt. Would need to know what car, or more impotantly what type of rear lighting set up the car has (all bulbs mounted on a printed board, or individual lamps with a common earth, or individual lamps with seperat earths) to be able to answer more accratly.

Any veiws expresed in this thread by me are purely from my own experience, and (sometimes) falible memory. Hope my comments help, but please don't take them as gospel.