Heater not working in Suburban

I have a 97 Suburban and now that the weather has turned cold, we've discovered that the heater isn't working. The rear heat still works,
and I've checked that the heater core isn't plugged (both hoses through the firewall get hot), but I get no hot air from the front. So I suppose that I have a bad switchor a control surface in the ducting is stuck. Are there other possibilities? Are vacuum lines involved?
Has anybody made this repair before that can give me some advice? It looks like I'll have to disassemble most of the dash to get to the heater controls and the blower box. Any shortcuts or advice would be greatly appreciated.
Also, are there parts that I'm likely to need for the repair from a dealer? I don't want to get stuck with my dash in pieces while I wait for the parts department at a dealer to order a switch or something.
Thanks for any help, Steve
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