I agree portable but in the mean time for a quick defroster fix use a harborfrieght #60525.
I use mine with just the fan (leave the Heat off) and my windshield stays clear (on my side anyway).
http://www.harborfreight.com/12-volt-au ... 60525.html
It has a toggle switch for "Heat" just "Fan" or "Off".
I think i got it on sale for $9.99, I did do a couple mods to mine to help it work better...
Yes that is a PC Power supply fan...the original fan is still on it but I just gave a little boost.
The heater part works but is very week.
You can also turn on the car heater and set it to the low 60's (I think someone said 61) and fan on low and it helps keeps the windshield clear and the range does not suffer much (if at all).
The cooler temps have affected my range also (70's down from 90's) but last night the temp only got down to the 50's and I had a 95 mi range this morning so I am hoping it gets back to normal soon.