Car Overheat, no heat coming in through vents

Hey everyone, this is my first e-mail. I own a 1996 Grand AM SE, 177 000KM, 2.4L engine.
Two days ago my car wasn't bringing any heat into the car, and my temperature gague was telling me it was going to overheat soon (not in the red. But close). Temperature outside was -20 C. I made it to work (15KM) and back (15KM), because it was a sunday. Prestone/anitfreeze was bubbling out of resevoir! And gurgling was coming from under the hood. I took it to my mechanic, still hot and full of that problem, he flushed out the anitfreeze, added water, tested the relays, checked fuses? He said that what he could see was that the cooling fan wasn't coming on, not relaying the signal to the computer. He suggested I take it to GM Dealer to run on their computers, to be sure. The problem didn't re-occured since I left my mechanics. I took it to GM 23/01/07, couldn't find the problem, and wanted to keep it over night. He told me that I had no prestone in the car, and that it would freeze overnight, I told him to get it from my car I have some spare anitfreeze? Kept it overnight, and now the GM DEaler is going to replace the Cap on the radiator. But they can't find the problem? Sounds like an air bubble to me. Can they charge me labor if they couldn't find the problem and any idea what this problem may have been? Thanks for your help in advance. I tried to be brief.