i am using a somewhat dated eberspacher D1LC.
Now in Ecuador we are finding that the unit is reluctant to work above 2200metres.
As here in the Andes we are often at 3000 + metres this is now an issue.
Not sure of specs for webasto.
Also, interestingly, I am currently travelling with a fellow in a landrover who has fitted an eberspacher Hydromatic 4, not as old as mine.
His unit works well (pre heating the engine and cab) up to 3500metres !!!
I am assuming as his unit is much newer and also installed in Switzerland perhaps he has some addtional 'flow' options that improve it???
Eberspacher says for my unit :
"Heating operation at high altitudes:
up to 1500 m: Unrestricted heating operation.
above 1500 m: Heating operation is possible during a short stay ie.g. crossing a mountain pass, taking a rest), If a longer stay is planned (e.g. winter camping), the fuel has to be adapted to the altitude. In this case, please consult the heater manufacturer for advice."