Newby with a Problem

Hello all
I'm currently rebuilding a 1990 200TDI 90 with new chassis and Bulkhead, now at the stage of trying to sort out all the various electrical circuits. There was no wire from the Diff lock switch to the Binnacle, I have proven the switch and fitted a new wire, the switch is also earthed, there does not seem to be a connection awaiting the other end. The wire should be Black/Blue, all the threads relating to this do not seem to be conclusive, any help would be most appreciated.
Paul West Cornwall.

Do you have any pictures to describe your issue?

Not a great deal to take a pic of really Rusty, the switch on the transfer box is set up ok to switch to earth with the Dif lever engaged. The Black/Blue wire is connected to the binnacle alarm light and disappears into the loom, I cannot find the other end of it to connect to the TB switch wire. Maybe if I cut the Black/Blue Binnacle light connection and ground it out to prove the light.

To conclude this thread, its so wrong to make assumptions, no bulb in the panel the reason I could not get a light, the Black/Blue connecting wire was found entering the chassis to feed the rear lights and going in the opposite direction to the Binnacle. All connected and with a bulb in the panel I now have a Diff Lock warning light.
Onto the next problem.