Hi all need some advice on worsening problem with fan blower on my A4 2.5 TDI Quattro sport .its 2001 plate.

Fan blower would,nt work when out side temp was below 0 deg.would only work above freezing put it down to the very cold and damp winter.
Then it stopped working on way to work but would annoyingly start working once parked the car at end of journey!! from january til now fan blower would work on way home from work but has now stopped working altogether.
was thinking it was some sort of temp sensor issue with heating control unit. Any advice would be appreciated