I didn't have time yet to get on all fours and sprain something else that is old and brittle testing it in the car under the dashboard, but I found this out while testing the unit at home:
My car doesn't have AC or climatronic, so it doesn't require a POT meter to be present in the V154 unit to know it's current position. The same unit seems to be used or still be used in many VAG cars, so this might be helpful to others.
if you have trouble with the ventilation loosing power, sucking up air from the passenger foot-well when recirc is disabled, you hear clicking or other noises from the device or when the recirc button turn's of pretty quickly, this unit might be the cause.
I my situation, there is no active monitoring or control over the V154 by any modules in the car. When your turn the contact on, it always drives the motor for 10 secs to the 'fresh intake' position, and when you press the 're-circulation', it reverses the polarity on the motor and does the same the other way round. It even does this when the module is not attached.
When they gears are lined up as described here above after the 're-circulation' position is met, it should line up the two tabs marked in green in the picture or on the other side. When the cog is moved towards position 'red arrow 1' and the two tabs meet, the cog is pushed outwards until it runs free (red arrow 2). Same goes for blue arrow 1 and 2 , but that is the other way round. If you hear clicking, noise or if the motor has a hard time turning or sliding the cog to position '2', it would need cleaning and lubrication of the motor/gears and especially where the large cog slides in and out of the unit/last gear. The large cog is held to the unit by tabs inside the hole in the last gear, which broke off in my situation, but that doesn't hurt any functioning when installed... I think.
If the sliding part is dirty or doesn't have any lubrication, it may finally jam the whole system or burn out the motor inside the V154. Note that a new V154 costs around 125 to 150 euro.
I'll be back if I managed to install it back completely into the car.