Odd scenario, I need help to understand. 04 BEW Jetta with manual. It's got a frostheater on it (goes on one of the hoses leading into the oil cooler) and especially lately that has been worth its weight in gold. Given the cold recently (we've been fluctuating between -10 to 10 *F), I almost always have it plugged in. The heater doesn't keep my coolant at temp, which I expect given the severity of this cold, but I can definitely see movement on the temp needle after a few minutes of driving. If I use the heater when it's 30 *F out, the needle will be around 140ish. The other day I had to start it in -5 when it hadn't been plugged it. Took a few cycles of glow plug and cranking it for a bit, but got running. Took close to ten minutes to get movement on my temp needle. Close to twenty to get up to temp, but no heat. Assuming the dial is accurate-ish, the car will always get up to 190 (needle is vertical) and then stop rising. That's what happened, fully warm coolant, but zero heat. None. Turning on the vents produced air current so cold it would numb my fingers in a few seconds. Next time I drove the car (having it plugged in), heat worked fine. I experimented with leaving unplugged for the next day, and same no-heat condition. So my question is, how is the cooling system working? Where is the temp sending unit that feeds the dash? I ask that in relation to, is it possible my tstat just never opened? I can't quite grasp how in one case I get heat and in another, I don't, when the coolant is at temperature in both cases.