I have been using the resistor approach and sensing the voltage when the LED is off and then that drops close to zero when the LED is illuminated.

There are 6 LEDs in play and 5 out of the 6 work flawlessly but I'm having problems with one and it is driving me crazy.

Intermittently the PIC reads the input as positive even when the LED is illuminated just on that one LED. Further information to set the scene:
This circuit is in a car
When the engine isn't running it appears much more reliable (suggesting perhaps noise or voltage related)
With the engine running, turning on the courtesy light will sometimes correct the reading (falsely reading until the courtesy light is turned on and then it reads correctly). This again suggested voltage but turning on something else like the headlights does not have the same effect.

I've tried varying the resistor value and putting in capacitors but has no effect.

Anybody have any ideas? I'm pulling my hair out here.