[Biturbos4] Lack of heat in B5 series car - any thoughts? (common problems, etc)

[Grant Lenahan]

OK, its a Passat. and it isn't even mine.

But a friend, who's a pretty fair mechanic, just bought a 2001 Passat  
for his son.  It should be very similar to B5 A4s and S4s in this  
regard, I assume.

One problem is that the heat doesn't work - it blows stone cold air  
(I'm told).  They know the history of the car, and the owner says it  
was working normally one day, and totally gone the next. We suspect a  
mixture (cold/warm air) stepper motor died or a connection to it has  
a problem.  But i've not heard many complaints about this.

Any wisdom? Common problems?  Things to check? Warnings?

Thanks much,

Grant