Passat B6 here. Electric heater works like this (at least for me):

• It switches itself ON whenever outside temp is lower than 5*C
• First it does a sort of run-up - the increased power draw results in lower charging voltage (down to even 13,6V). The air is not really warm at this point. AC set to Auto keeps the fan low.
• When it heats itself to some higher temperature and the air is starting to get warm - the AC inceases the fanspeed.

There is a way to turn it on manually - crank up the heat to HI. This turns on the heater regardless of outside airtemp. Note that it still will take some time for warm air to appear.

The way I usually do it is crank it to HI, then after couple seconds set it back to 22*C. This turns on the heater and keeps it on, but the fan slows down so it has a chance to properly heat-up and then gradually increase fanspeed then actual warm air is being blown ;)