Not meaning to rub it in alfapash but I wired up a separate button on the central console by the handbrake as I got fed up fishing around in the glovebox for the switch this evening
However, the reason I'm posting is to perhaps save you some time. To diagnose your problem I would:
1) Remove the 5 allen bolts from the inside top of the glovebox.
2) Remove the 3 allen bolts from the bottom rear of the glovebox (underneath the glovebox and closest to the front of the car) whilst supporting the glovebox with your other hand.
3) Lower the glovebox. You will see there is a cable connecting it to the car on the RHS. Press in the little plastic lug on the connector, on the end furthest from the cable and it will pull off.
4) Take a paperclip and short the 2 pins at either end of the connector on the end of the wire. If the boot opens, the problem is the switch.
5) If that doesn't work - see if there is 12V between those same 2 pins. If not suspect fuse/wiring/burnt out relay coil (unlikely). If there is 12V then it's probably a sticky relay, wiring between relay and lock or problem with the solenoid on the lock.