Hi mate, I know I've posted a few times and explained exactly how I did it, but I can't seem to remember off the cuff now.
There was no relay used, just three wires I, and an inline fuse. I'm searching my old posts now for you, hopefully jog my memory.
Personally my switch only lit up when you put it on, but if you were that worried about MOT, just get a switch that is illuminated 100% of the time. (or you could use an illumination live off of the electric window switches (on the inside of the car side as opposed to in the door). That's easy enough to find with a tester.