Rear Heat

I have an '03 Tahoe LT w/rear heat that doesn't seem to get hot. I talked to a mechanic and he said it might just be a resistor or switch that is out. Has anyone else had a similar problem? I just bought it this summer and obviously didn't try the heat until it got cold out. Let me know if you can offer any advice

Common problem on Tahoes.
Climate control: Low output of the rear heater in very cold weather is due to a restrictor in the T-coupling. A revised coupling is offered. (2000-01)