doubt cig lighter heaters will do much for ya. I think they are more for supplemental heat than anything else. On a 50 degree day they would prolly be fine... but if its 35 and you have frost on the windshield, it aint gonna cut it.
I recently got a mojave style heater from summit. It cost $159.99. It supposedly can put out up to 28000 BTUs. I got the dual vent style. Im gonna run up a hose for defrosting capability when/if I ever need it but other than that it looks like a sweet little heater. Its small, made of metal (no lame plastic crap here!), it comes with a nice 4-position switch, mounting brackets, all the wiring you need and clear directions too. The intake hot water hose inlets are also the same size as the stock ones on my K5. I'll post some pics when I get done with the install.
j