1999 Ford Windstar with no front heat

My 1999 Ford Windstar only has heat in the rear. I have tracked the problem to a little white box in the dash, behind the radio. When you slide the temp selector from cold to hot, or vice-versa, the little box clicks like it just does not have enough power to do what it is supposed to.

This little white box has a small knob in the lower right hand corner with a recessed square hole. This knob feels like it has a shaft that comes out of the back and goes through the firewall to the engine compartment. There is a small black box and a small black plastic canister (about twice a big as a film canister) in the engine compartment, attached to the firewall, just opposite the white box. I assume the shaft from the white box goes onto the black box in the engine compartment? Is this correct?

I have also noticed if I move the selector from cold to hot, I can help the white knob rotate by inserting a screw driver and turning just a bit. This will engage the heater and blow hot air unless you move the selector switch back to cold. Then the white box clicks and only blows cold air.

I unscrewed the 2 screws on the white box assuming I could removed it and replace it, but the shaft coming out of the back seems to be attached and will not come off. I did apply just a little pressure to try and remove the white box, but was afraid to pull to hard. Any ideas as to how the white box comes off?

There are three screws.

There are three screws.

Thanks. Don't know how I missed the third one. When I take all 3 out, will the white box come off easily, or should I expect to find it is attached to wires or vacuum hoses?

Thanks. Don't know how I missed the third one. When I take all 3 out, will the white box come off easily, or should I expect to find it is attached to wires or vacuum hoses?

There's one elect connector. Fairly easy.