Near the end of November last year, I took my 1994 Ford Ranger XLT six cylinder 3.0 with 160k miles to a shop I have used many times to my satisfaction, to have a new water pump and an A/C kit installed.
But ever since the install, my heater no longer blows hot air. (Before this repair my heater worked EXTREMELY WELL.)

I did not notice the problem Immediately because I wasn't driving the truck and the weather was not cold. But a couple weeks after having the new AC installed I noticed the sound of rushing water behind my dashboard when I first start the truck and turn the heater on. So I took it back to the shop. They told me the issue is that my radiator was gunked up and would need a flush. (In fact they pointed out that I would need second flush in a few weeks when they first installed the AC.) The water rushing sound behind the dash did indeed go away after that visit, but I eventually noticed that my heater was not producing hot air no matter where I put the settings. But strangely I did once noticed that it was blowing hot air in the OFF position. WTFudge??

Long story short -- it's now March, it's still cold out, my fingers and toes have been freezing all winter, and i'm sick not having heat. (Said the guy who did without AC for the last two years!)

I don't live near the shop anymore which has prevented me from dealing with this sooner. Nevertheless, I'm finally taking it back next week. But I want to walk in with some knowledge about what is going on, as well as a possible diagnosis/solution. Because I'm convinced they mixed-up/crossed-up something under the hood during the AC and water pump install that is preventing my heater from blowing hot air. if not, I'm wondering if it could be the thermostat (I asked if it should be replaced when they did the water pump and AC and it was included in the quote.) Or perhaps a fuse or something fairly simple I could fix on my own that would prevent me from the 70 mile drive and hassle of going to deal with the shop.

Any help on what the issue could be is greatly appreciated. Thanks

__________________
1994 XLT 6cyl 3.0 -- Deal with it!