I posted another thread on here regarding general winter driving tips but I wanted to address this one by itself. After owning our Prius since May 2008 and putting about 12,000 miles on it 133k total now, we got our first real taste of winter this weekend and cold temps (-15F right now, expected to dip down to almost -30F tonight). The interior cabin heater output cannot output nice hot air now because the engine just can't warm up enough. I tested blocking the radiator tonight using 2 pieces of cardboard, however, short of taking some parts out, I cannot squeeze one nice big piece of cardboard in there. Have people been blocking their whole radiator, part of it, none of it andonly blocking the vents in the grill/bumper (or both) or anything else (hotter thermostat, etc...)? If I remember right, G2 owners use pipe foam insulation on the grill slots, but I haven't heard of G1 owners doing that. While I wouldn't normally mind lower heat output, I do mind it in this car because the interior windows fog up easier than I like (and frost up quick at -15F), and MPG obviously suffers if the engine is not warm enough. Toyota's HVAC systems are normally very well designed and move a lot of air. The G1 seems to be a little bit below average. I need to check the cabin filter again before I officially blame the car, but when I checked it 7 months ago when we got the car, it was clean. It's so cold outside that the radiator hose on the driver side is only luke warm after the car has been driven a while, and the radiator hose on the passenger side is cold. I blocked about 3/4 of hte radiator and drove about 10 miles and it seemed to help a little (and the little blue "cold" light on the dash went out much faster). Anyway, just looking for any tips. I had almost thought low coolant or a blocked radiator, but the coolant level was fine, and if the radiator was actually blocked, one radiator hose would be extremely hot (as opposed to just luke warm). So, it looks like I found one (and only one) thing I don't like about our G1.