Heater air door not working, help!

have a 95 Sonoma PU with 4.3 l v-6, both heater and A/C.
Everything was working fine. Now the "air door" that switches from "floor
air flow" to upper dash vents no longer works. ALL air flow in out the bottom vents, A/C, outside vent and of course heat.
This PU has lots of relays under the hood next to the heater, A/C housing and some in the glove box, could one of them be bad?
I have checked fuses and all are okay..
Help, suggestions Please?
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