If the same as my 2002 4/4 it is on the bulkhead tucked up on the right hand side (UK drivers side) near the top and screwed to the woodwork under the windscreen (from the inside of cockpit). I think I had a similar problem and looked at the module (black box approx 5cm square)and noticed it had got damp so used cleaner on the circuit board and seemed to solve it. When I put it back I put a plastic cover in case more moisture got in.
I also cleaned the contacts in the switch, it comes apart like Lego and used Switch Cleaner on that too. (I would try that first to be honest)
