Here is the switch before removal.  It is bolted to the side of the gearbox where the gear selector arm connects to the selector shaft.  You can see the selector cable connection clearly in the photo above.

The photo on the left shows the nut removed that fastens the selector cable to the selector arm.  In the photo below, the selector arm has been remove from the selector shaft.

There are two 10mm A/F bolts that hold the switch in place.  These bolts were removed and the switch pulled off the shaft.  It took a bit of effort to prise the switch from the shaft due to a small amount of rust on the shaft.

Here is the selector switch removed from the car.  Upon close inspection I noticed that three of the aluminium rivets that hold the two sides of the case together had corroded and allowed the case to slightly open.  This is no doubt where water has been getting in.  I was a little concerned about what damage I might find inside.

I drilled the heads off the rivets and opened the case.  I was expecting to find lots of corrosion, but fortunately it looked pretty clean.  I cleaned the switch contacts and and checked that all of the switches operated correctly.  I also checked continuity of each of the wires.

I cleaned the case and replaced the rivets with small bolts and nuts.  My Haynes manual suggests that a special tool is required to align the switch when when re-fitting.  However my first attempt at re-fitting was successful by simply getting someone to watch the dashboard display while I positioned the switch. 

Just recently the auto box on my 1999 Land Rover Discovery 2 TD5 has occasionally gone into ‘limp home’ mode, which is indicated on the dash by two green flashing lights.   While in this mode, the gearbox will only use 2nd and 3rd gears which makes pulling away from standing still awkwardly slow.  The first time this happened was just after driving through a large puddle in heavy rain and since then it has mainly happened in the wet.  While it is in this mode, the indicator that shows which gear is currently selected goes blank.  Stopping the engine, moving the auto gear stick then re-starting the engine usually gets it working correctly again.   All these symptoms add up to the gear selector switch being faulty.  I decide to take it off the car and check it out.

My Haynes manual described the removal procedure which included the need to remove the front section of the exhaust pipe.  I decided to have a go without removing the exhaust which made it a little awkward to get to the switch, but it was still possible.

It has been about three years now since I repaired the switch and I am happy to say that the fault has not returned.