My 2002 110" TD5 is having cooling issues.

When I turn on the heat in the cabin there is no hot air coming out it's cold. And the reason for this is that the coolant hose going in to the heater matrix is cold even though the engine is at the right temperature. If I drive 90 km/h in 4th gear there's hot air coming in to the cabin if I shift to 5th gear the hot air disappears. If I drive in the city there is also no hot air.

The car runs just fine and the temperature gauge is at the right place all the time.

If I use the Webasto car heater while parked, hot air is coming in to the cabin and the engine gets hot as usual.

When I look at the expansion tank I can see that coolant has been leaking through the cap (the cap is new). It seems to be from over pressure.

I have bleeded the system a thousand times.

The thermostat and water pump has been replaced with no luck. I have taken the heater matrix out and cleaned it. I have checked the hoses to and from the heater matrix and there are not blocked.

A pressure test of the cooling system showed no leaks, and the coolant is checked for exhaust gasses and there was no issues here either.

What is my next step? I have no idea where to look now......

I presume you're verified that the cables are adjusted correctly so the heater box flaps are opening fully?
The cables can slip through the clamps and reduce movement.

Yes the cables are adjusted correctly