Rear Defrost for Chevrolet

Anyone have an idea on how and where to hook up the rear defrost on late model Chevrolet / Chevy trucks and SUVs? I've had 3 people ask for that feature but I am scared to go fooling around directly with the climate controls. I was hoping to catch this feature someplace else. If you could talk me through it, I would appreciate it. I've had requests for the following vehicles:

2003 Trailblazer LT and LT EXT

2004 Tahoe Z712005 Avalanche LT Z71

Thanks everyone for at least reading it over

You can do it!

Some maufacturers claim that their data interfaces may do this, but I have never confirmed this. The best way to do this is to use the starter to activate a relay which will duplicate hitting the defrost button or switch. This involves soldering wires on the switch contacts inside the heater controls directly on the circuit board. It can be done but, not quickly.sparky