i'll just change the bulb...

I noticed the diff lock light wasn't working recently - no problem, must be a blown bulb. Nope. No problem, just dirty contacts at the light unit. Nope. No problem, must be a bad connection at the switch wiring. Lots of dirty fiddling later - Nope. Switch out (if you use a 16mm deep socket and a flexi connection, it's easier) and the cause "found" - a bad connection at one of the switch tabs, where it's riveted on. No problem, I'll just clean it. Tab broke off. No problem, I'll just solder a wire onto the brass switch connection. Soldered on OK, but - Nope!
New switch required. Bugger.

I have similar...just taken the inspection panel out and found the switch disconnected.....probed with meter and the switch works perfectly...but I'll be damned if I can find the wires the switch is supposed to connect to!

There should be a short black wire connected to a bolt on the transfer box , and then a single wire (probably in a black cover) leading to the loom.
I've had a stroke of luck - found a spare switch in my biscuit tin of odds n sods.