Originally Posted by wbrian63

Figuring out which lamp of many on a car is the culprit can be challenging. To help narrow your search, make careful note of when the indicator light comes on. There are 4 different possible situations that will trigger the indicator light - 1) Parking lamps - these are the lamps that illuminate when you turn the headlamp switch to the first detent. 2) Headlamps - lamps that illuminate when you turn the headlamp switch to the second position (I believe this would be confined to just the left & right headlight only). 3) Brake lamps - only on when you depress the brake pedal. 4) High-beam headlamps - only on when 2 above is true and you select high-beams from the turnsignal stalk.

When exactly does the indicator lamp come on? When you switch on the parking lamps, or not until you turn on the dipped headlights?

The answer to this question will point you in the direction of the solution.

If the indicator comes on when you switch on the parking lamps, then the problem lies with one of the bulbs in that circuit. Front parking lamps, rear parking lamps, license plate lamps, daytime (city) lamps - if car is so equipped. Some cars also have lamps on the front fenders or possibly on the exterior rear-view mirror housings - don't forget those.

If the indicator comes on after you switch on the dipped headlamps, then the problem is with one of those two lights.

If the indicator comes on when you step on the brakes, then the problem is in one of the bulbs that provide brake lamps in the rear.

Note that the system is sensitive - other posters have noted issues with corrosion, which is a likely cause if present. On my car, the PO wasn't too careful when replacing bulbs. I found the wrong size bulb in one of the parking lamps in the rear - the base fit fine, but it wasn't the right wattage (although I could see no discernable difference when viewing the car from the rear). That fixed the indicator that lit when I switched the parking lamps on. However, I still got a light when I stepped on the brake. All bulbs in the rear lamp assemblies were verified to be the correct wattage and no corrosion was present. Finally discovered that the PO had replaced the high-mount stoplamp bulb with the wrong size.

Be persistent and logical in your approach to the problem and you'll find your bad bulb/connection.