Rear diff lock wont engage? :(

[Gaz]

a couple of days ago i used the center and rear diff lock to climb our hill due to possible ice. (better safe than sorry)

then i disengaged when i got to the top. but the following day i tried it again and the rear light wont come on ???

i remember hearing a 'clink' type sound halfway up the hill on the least steep part, I haven't had a chance to have a look underneath the car cause its dark when i get back from work but could it be fu8ked? I hoping its just a vac hose issue, but i don't see why it would be if it worked before and the center still engages as normal.

any ideas?