Yeah,those 12V heaters are only 150 watts,a light bulb puts out more heat than those will!...they might melt a peephole in the frost if you place one right against the windshield,other than that they are a joke..
I've used a 110V house type ceramic space heater in my van and Suburban before when it was about 20 degrees out,while I was inside them working on them,and even that barely helped warm it up at all--my "sunflower" propane bottle heater wasn't much better either,and that can suffocate you.
I have no trouble keeping warm in my Suburban or van with the stock heater,as long as the engine reaches full temparature--I use only 195 degree thermostats,and if need be,I'll put some cardboard in front of the radiator to boost the temp up a bit when its extremely cold (like under 15 degrees)...
My 6.2's put out great heat as long as your driving,or putting the engine under a load plowing,etc,but if you sit still and let them idle,the heater will only put out lukewarm air--a diesel trait that all seem to have,they just run cool at idle,so little fuel is being burned..
I put a rear heater in my '72 K5,I got it from a school bus in a junkyard,it went under one of the rear seats--I put it in a metal milk crate and that thing cranked some serious BTU's,I could melt a foot of snow off the roof in 15 minutes after the engine warmed up--no more brushing it off...
I also made a heater from a used heater core and blower motor from a truck I scrapped,I mounted them in a plywood box about 16" square...I had that in my van one winter,it worked very well...but I removed it when I decided I wasn't going to drive it in the salt,it took up quite a bit of room too..