Haven't had time to do more work on the heating, but I did a long drive this weekend to a show. 150 miles out Friday, 150 miles home Sunday.

50 miles from home the heating started working with a high burst of air from the fans (including a cloud of dust...), and surely heating now worked. No way to turn heat of, but could this be the potentiometer adjusted out of useable range ?

Another thought: Could low voltage/bad earth cause the climate control to fail ? System has now started working twice during long drives, where battery charge probably is better than on shorter drives.

Final observation: Fans almost works as expected when the heat works:
DEF: defrosts
HI: Blows high
Auto: Blows not so high
LO: Blows low
OFF: Blows more than low, but there is less air to the feet. Not really off...

I found that max heat and thereby open windows tends to lower my speed on motorway...