@tottot and
@Turboman have given you the answer.
It's a medium task, takes a bit longer than you'd think but well worth doing.
Take off the vent on the NS wing and unscrew the plastic tubing that goes towards the heater.
It starts out vent shaped and ends up a 5" tube.
There is a foam seal here that is often disintegrated.
Remove the two hoses that go from the back of the head to the heater and look a hose between the two connections on the engine - this means you can still use you landy whilst you sort the heater.
You may need to bleed the coolant as you will have lost some taking off the hoses.
Unclip the two control cables (one on the top and one on the side near the bulkhead) - mark the cables and do not lose the steel clips ....
Undo the electrical connection.
There is a seal between the heater and the bulkhead, this will also be disintegrated.
There are 4 bolts holding the heater to the bulkhead, with these undone the heater 'will' pull out - I can't describe how you do it but it 'will' pull out.
Here's a usefull pic ....
https://www.lrparts.net/lrc2379-sea...-hand-drive-up-to-2006-includes-fittings.html
Once it's out (as
@Turboman says) you drill out the rivets, take the box apart, and look nat how the air circulates around the heater matrix or is diverted away from it (hot/cold flap).
You will see if you matrix is leaking or damaged straight away.
New ones are available - there are two types, check tyhe shape of yours and the in/out pipes (straight or bent) - you will probably just need to repack the matrix so that the air flows through it nor around it.
Clean it up
Put it back together with some sealant and 3 million rivets (not that many really).
Clean up the bulkhead and the hole into the cabin ducting.
Refitting really needs two people and is a sonofa bitch, the bulkhead seal seems to be to big - but it isn't - it'll never fit in - but it does.
Taking the wing off seemed like a good idea when I was doing mine, I was seroiusly considering it - but suddenly - pop - it was in.
Refitting the cables and the hoses is straight forward - again you need to bleed the coolant into the heater - I poured water into the heater hoses until if flowed out and then connected to the engine
Mine went from pi55-poor to not bad at all, bordering on OK, in real terms it is much better, chalk and cheese
It takes a mile or two to warm up and after 10 mins or so the screen clears nicely.
Including getting the seals, painting it all up, refitting etc it took a couple of days spread over a week.