get these mics (or similar...they don't need to be super high quality, just very small with a long cord.):
http://www.core-sound.com/lcmics/1.php
plug the mics into one of these (any decent digital recorder with a 'mic in' option will work):
http://www.amazon.com/Edirol-R-09-WAVE-MP3-Recorder-Black/dp/B000FPQFKO
put the recorder (size of a box of cigarettes) in your pocket, your purse, whatever. clip the mics under your collar so they can't be seen, or to your glasses if you have long hair that will cover them. just before you get out of your car, turn the recorder on.
Easy peasy, lemon squeezy.
(only recommended for states where it is legal to record a conversation like this of course.)
(And actually, the recorder above is good enough that it would record without the mics, just sitting in a purse or in a pocket...the mics would just help get a better recording. I've also had success with a camera to record the audio, as another poster said.)