Is there a side panel at the end of the dash that can be removed?
It may only be able to be removed from the rear, behind the dash - maybe able to view it better if the glovebox / lower dash is removed
The first place I would start is the battery condition and security of connections, etc
It is possible it could be reading the wrong voltage if it needs to be programmed in
Try eBay as a starting point
https://www.ebay.co.uk/b/bn_1838652?toolid=20012&mkcid=1&campid=5338906902&siteid=3&mkrid=710%2D53481%2D19255%2D0&customid=
I don't believe it will need coding if it is a direct replacement part
The only time it would be necessary is if it is part of the ADAS system on the windscreen and then it would need to be calibrated
I would consult a campervan specialist as there will be items such as charging relay and switches that would need to be added to ensure it is safe from overcharging, etc
It does sound like the resistors have burnt out and if speed 3 drops out it only leaves speed 4 directly running through no resistors.
Here is a link to the part which is incredibly cheap - fitting may be a little awkward but easy enough to replace
https://www.ebay.co.uk/itm/370625832914?fits=Car+Make%3AVW|Cars+Year%3A2012|Model%3ATouran&_skw=2012+vw+touran+heater+resistor&itmmeta=01JJP7V9E9KSY7N66MPF2HBNVX&hash=item564b0567d2:g:8cEAAOSwhilZaIht&itmprp=enc%3AAQAJAAABAHoV3kP08IDx%2BKZ9MfhVJKn%2BBd4b61r9iUk5%2Fni3yiSZj5X%2Be0hU4IK8WwT8qKVFWC%2FkcRTTnAu1395RkkrqdEk5vHPx3AgcWnYw2L84rYqk4713Rm208dVjfWBPVzexWeWvOZynUIAxkgiiQGgouYywmmX2%2F1Zq5cUYWsislXEsdV8WYGNNsvEIwYjyqNH38aUVpnxLXG4tb7oB5FkIm%2BHU98TCM%2Bu3NhYp7erJn2NK6bU5Jr2l%2FqDdwZYtlcaCMNkUWRGU%2FwGIhiTeAR5xe1JdxorzqcZU9dgdTheY4rv74Mo4AZDqyds3btv3FUXJplfQ3Hdu8mDZk1N2c8yqaN8%3D|tkp%3ABFBMpJftx5Vl
It would be interesting to know what they find during the service - whether it is a mechanical or electronic issue.
Maybe just needs some recoding to restore the correct settings