I'm in the middle of replacing mine at the moment. Mine is a 1993 T4 1.9D.
These bulbs are not designed to be replaced as the heater control unit is sealed together, but this isn't going to stop me !
To remove the heater panel pull the slider control knobs off with some pliers (gently does it). There are 4 retaining clips on the fascia that need pushing down to remove (2 on top 2 on bottom). Pull the unit away and unplug the blower control dial socket and the light socket.
Once removed there is a white colored housing on the reverse that needs to be prized off. I used a craft knife and a small screw driver to prize the edges apart. Its not easy to do and I did damage some of the edges but nothing that will be seen when reassembled.
Once you have got the back off you will see 1 bulb behind the light terminal. After a bit of head scratching I decided to push down on the 2 terminals with a screwdriver and this forced the bulb unit out, hurray!
I'm going to replace the bulb with an L.E.D. so it should be brighter as many people say the original is pretty rubbish.
I haven't got the replacement bulb yet but I think I will need to solder a new one in. May need to alter the bulb housing to allow wire access.