Why does 2005 VW Passat ac blow hot air when idle but cool air when in motion?

I just had this happen on my 2004 Passat wagon. Dealer found that the cooling fan motor has failed and the AC goes into bypass becasue engine compartment temperatures are not controlled when idling. Once you begin driving, enough cool air enters to let the AC function again. Replacement fan motor apparently involved removing the front bumper! $700.00 KWB