The brief
The Golf's cabin airflow had gone weak on every fan setting. Turning the dial up to high made it run a bit louder but barely changed the air actually coming out of the vents. The two together, weak across the board and no real jump between low and high, point at the blower motor itself rather than the control side.
The blower is the fan behind the dash that pushes air through the cabin, whether you're heating, cooling or just demisting. As the motor ages its bearings drag and its brushes wear, so it spins slower and slower for the same command, and eventually a winding can go open and it barely turns at all. That's the weak airflow and the lack of difference between settings, and once a blower's at that stage it's a new motor, not a repair.
The diagnosis
The scan logged an open-circuit fault on the front fresh-air blower, and the climate control side checked out: it was sending the right speed commands, and the resistor and control module were fine. Bench-testing the blower motor at full voltage showed it turning slower than spec and pulling more current than it should, with the bearings dragging and the brushes worn.
That made it a motor replacement. A worn-out blower only gets worse, so a new one going in.
The work
The under-dash trim came off to reach the blower assembly, the failed motor was dropped out of its housing, and a new VAG-spec blower motor fitted in its place, the harness reseated. With the area open anyway, a fresh cabin filter went in too, since the old one had been collecting dust for a long time.
Then the fan was run through every setting to confirm it was pulling strongly again.
The outcome
Strong airflow on every fan setting, a clear step up from low to high, and the cabin smelling fresh from the new filter.
The Golf went home with the climate control behaving the way it should. A tired blower makes the heater, the aircon and the demister all feel weak even when they're working fine, so a new motor brought all of it back at once.