This might help you trouble shoot your wiring, at least you should prove that works first.
From the Haynes manual for your model (Book 1081 Published 1999) Diagram 43 and 47
The principle is that the bulb (named K7) always has 12V on the BLACK wire when the ignition is on. (But I can't tell where this comes from). Check the BLACK wire has 12V.
The GREY wire is then grounded by either the handbrake switch (F9) or the brake fluid level sensor (F34).
Check using your multimeter on OHMS or CONTINUITY that the GREY wire connects to earth when you pull the handbrake up (with the ignition on).
If you suspect you handbrake switch, you could remove the cap from the brake fluid reservoir with the cable still connected, this should also light the bulb/led.

(The fuse on the right is for something else)
The colours might not be exactly the same. One of your handbrake wires might be brown (Meaning always earthed) and the other might be brown with a green stripe (Meaning switched to earth)