Planning the next
AO Silver Member
Join Date: May 2007
Location: United Kingdom
The LED is fed from the output of the rear fog device ( it's more complicated than a simple relay ). If the actual fog lights are OK, you can rule the switching device out.
First thing I'd do is measure the voltage at the instrument pack. You need to measure at connector 'A' ( the connector is probably white ), pin 3. The wire, to check you have the right one, is plain grey.
It's unlikely to be anything other than battery voltage, but if it isn't, then you've narrowed the fault down.
If you do get battery voltage at pin 3, you're probably looking at a dry solder joint on the PCB or a failing LED. Less likely is a failing current limiting resistor.