Do you have air conditioning?
Turn it on.
You only need a little heat. And if a 2.9L isn't putting out enough, the question isn't how to defog the windows, but how to fix your cooling system (and probably the cracked heads).
If the airflow is weak, dig out all the leaves from the heater/evaporator box.
The idea that a 12V defroster is going to do ANYTHING (aside from MAYBE blow a fuse or two) is less than sane. Most hairdriers (which approximates what you're trying to do) are 1500W. Exercise #1 is figuring out how many amps you're about to try to pump through your electrical system. Exercise #2 is to find out what your biggest fuse is. If you can't do either of these, do NOT F with your electrical system or you'll break it.