What causes the AC fan to blow at top speed but not much air to come through the vents on a 1997 BMW 328I?

This here can be a number of things, first of all if your blower is running in only high speed, i would first try to put in a new final stage blower resistor, its located behind the center console on the drivers side, actually right to the right on the gas pedal, also if you are not getting much air to come through the vents i would also try to replace the micro air filter, which if never changed can be clogged causing air restriction, this is also located behind the center console, easy to get to if the glove box, and lower air duct is removed on the passenger side. try doing those simply task first, if still nothing is working, it could be defected air flap in heater box, or fautly ac control module. good luck