hi I have a rendezvous 2003 and I just change the heater core and now I'm having problems air blowing on the passenger side in heat on the driver side but only when you have it on defrost
Sounds like the blend door(s) are not opening or closing all the way. When the core was changed, the door might have something in it's way, like a wire from under the dash. If you had a mechanic fix it take it back and have him take a look at it. He should fix it at no charge, if he's any good.
Search Buick Rendezvous Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Buick Rendezvous Experts