Let’s start with a “how to” guide and hopefully your questions will be answered. That way we can assist any other members also wishing to complete this task. In the cab of your VW there will be three levers; one blue and two red. The blue lever controls the cold air intake, on and off, the left hand red lever controls the distribution of hot air between the foot well and the windscreen and the right hand red lever controls the amount of hot air entering the cab, location defined by the left hand red lever. It is the right hand red lever that you are describing which connects to two cables which travel the length of the van to the heat exchangers at the engine. These cables are readily available from VW parts suppliers and are relatively easy to replace, although you are experiencing the dreaded rusted conduit tube! Before work commences, it is advisable to jack the front of the van as high as possible (that you can leave for a day or so) and spray penetrating oil down the conduit tubes and leave as long as possible to allow it work. This should make removing the cables (both inner and outer sheath) a much easier task. To gain better access to the cables, and cab area, it is advisable to remove the steering wheel and is done so using a 24mm socket or spanner. To gain access to the nut, use a small flat bladed screwdriver and prise up the black plastic cover, not the aluminium ring around the cover. When removing the steering wheel, loosen the nut but do not remove it yet, give the steering wheel a few tugs to remove the wheel from the column, doing this with the nut loosely fitted will stop you smashing yourself in the face with the wheel when it gives way, (TRUST ME!)

If you can avoid removing the part of the dash where the coloured control levers are located,
I would. You should be able to reach under and disconnect the cables from levers in situ. They are retained by two clips and the cables loop over each other with a spring clip. Note how it fits before removing (take a picture). Follow the cables down 150mm and you will find the cable outer sheath is retained by a spring clip. Moving underneath the van, disconnect the cables from the heat exchanger flaps (8mm spanner) and pull the cables clear of the flaps. The nuts are prone to corrosion due the heat and exposed location, so it is worth soaking in penetrating oil (get a cuppa and come back) before removing to avoid damaging the flap on the exchanger.

Staying under the van, head to the front and find where the cables enter the conduit. Now pull the inner and outer parts of the cables back through the conduit, this is where you may have trouble if they are seized, but hopefully the penetrating oil has done its job. Many people have trouble at this stage, but do find that brute force and ignorance does get the job done eventually. Make a note of which cable comes from which side as they are different in length, you don’t want to come up short at the end of the job! At this stage, if you cannot remove the cables from the conduits at all, you could leave the cables in place, trim them back and fit an alternative conduit directly next to the adjacent one. Not ideal, but a potential fix. This could be done using some kind of plastic water pipe and cable ties, but I have never attempted it. With the cables removed from the conduit it is time to refit your new cables into the existing or replacement conduit. The passenger side cable has a metal rod bent at 90° on the end and the driver side cable has a loop made from the cable itself. Prior to fitting the cables, it is advisable to grease the inner cables as best you can and also grease the outer sheath part of the cables to prevent them from seizing within the conduit, should you need to replace in the future.

From under the van, push each cable through the conduit until it reaches the heat exchanger. The opposite end is then passed into the cab area through the grommet in the floor and reconnected to the heater control levers. The retaining clips on the levers are very tricky to hold in position whist you tighten the screws, but keep persevering and you will get there.
If you have it, another set of hands to assist
you may help at this point 9and probably another cuppa!) Now that you are done in the cab, move the RH heater lever to the down position and return back to the rear of the van. Connect the cables to the heat exchanger mechanism, making sure that you replace any grommets from the end of the conduit. Move the flaps to the maximum open position against the spring pressure and thread the cable through the retaining nut. Complete for both sides. Now check that when the heater lever is moved up, that the heat exchanger flaps close. An assistant is handy here too to save keep getting up and into the cab and back down on the floor again! If all is working well, the job is complete and you can refit the steering wheel and any underneath panelling removed for access. Now drive and enjoy your heating in the
colder months and ability to turn it off in the warmer ones!