You want the mic to measure about where your ears would be, so like u said, a few inches off the head rest.
Audio tools will be a decent start but if you want to get the most out of your DSP, you will need a program that can smooth more than 1/3 octave. 1/3 isn't really enough to dig into the problem spots. You can probably use that mic with a laptop and get REW. It's a free program that will allow you to see what is really going on.
You also want to average the response. If you measure where ur right ear is then where ur left ear is, you will get 2 different responses. With REW u can take multiple measurements and get an average response. With audiotools, u cannot.