light level sensor

My warning/trouble light goes on and the indicator indicated the light level sensor is the problem. It doesn't always go off; today I drove an hour before it came on. Took to the dealer and they qouted $500 for a repair.

Questions are: where is this sensor located? Is it part of the whole headlight assembly.

Is there a way to repair this without a new part.

Is there a place to find a used part.

Is this a repair a novice could do?

Thanks for any help you can offer.

Mark Cape Cod Ma.