A photo of the dash HVAC control would help us help you. When I thought mine was bad, i discovered it was a panel made by Evans Tempcon.
You might check the link, see if you see a matching panel. My panel, RV218416, was also used in Dodge pick up trucks, among others. I got a replacement from Ebay from a salvaged truck. You could also contact some of these RV salvage yards
, and see if they have the panel.
If, as your mechanic tells you, it's a faulty switch in the panel, you should be able to remove the panel or the electric connector on the back of it and use a jumper wire between the two contacts to see that it truly is the panel switch. I'd do that before just replacing the panel.