My GPS is mounted in a bracket on the dash, no pouch or heater. I have a small digital thermometer mounted just below it on the same bracket. Once the sled is warmed up the temp usually stays around 40 - 45 degrees except on the coldest days (like 0 F or below). I don't leave the GPS there overnight or during meal breaks. I haven't had any problems with the cold affecting the GPS operation.
It was -10 Thur. in the UP and my Maridian would not work, I know could of put it the map bag with a "throw away hand heater bag" to get it to work. So I'm going to assemble some type of heated glove so it works when in my powered holding bracket.
StudDog where did you get your heated holder? I've searched the sites you mentioned and did a google and no luck. My 1st attempt will be the neoprean cover from Magellan with a grip heater tape ( maybe 2 ) on the inside. shane