Hi fella, I am sure more qualified folk will be along soon enough but I can share my recent experiences
When I first bought the 110 nothing Heater related really worked....the problem as seized control cables. Pulled the dials and used plenty of spray grease down the cables and around the switch levers (take care to avoid electrics)...with some wiggling to free up all working in minutes
My heater packed up a little while ago. Had fans but no heat...solution replace heater matrix and seals simple enough job for any man with basic mechanical abilities
Next issue came along was that my heaters suddenly started blowing 10amp fuses every time they were turned on. Very frustrating. Happened recently amongst the cold snap too no obvious wiring problems found and after a little help from this forum found some good pointers. Initially I thought the heater relay was chuffed..I have another sat here now waiting for install that came via eBay and increases the speed of the fan on the 1st click (upgrade if you will) Turned out however that wasn’t the problem...on my 200tdi the air intake is on the left wing. The motor for the heater fan is inside the air intake at the bulkhead end. There is a hole and a funky “ducks bill” shapes grommit that is just in front of the fan. When it rains or your wading and water gets into the intake it’s supposed to drain through that grommit. Mine was blocked with ****e and hence after a couple of days of biblical rain the air intake was full of water and when I switched the heater on the submerged fan was tripping the fuse...removed the grommit, drained the water, dried the fan with the Mrs hairdryer and we are now warm as toast
If none of the above work you may have a knackered motor...quite possibly from an episode like the one described above. If you pull the heater box and either test with multimeter or run a 12v battery to it you will soon now
good luck