My first time masturbating in public was also the first (and only) time I did it in my mother's car. I was about 13 years old. She was headed to the grocery store and, being bored, I asked to go along. Once we got there, I wasn't so thrilled about the trip anymore, so I told her I'd wait in the car. I knew a trip to the store to 'pick up a few things' meant she'd be in there at least 30 minutes.
The idea to finger myself happened pretty quickly after she left. She had a station wagon with vinyl seats and tinted windows. I did a quick check and didn't see anybody lurking about the car, so I pulled my jeans down around my ankles and spread my legs as wide as I could.
Of course, being there where just about anybody could walk by and look in was an incredible turn on, so I was getting wet without even touching myself. My technique at that point pretty much revolved around rubbing my clit. I hadn't yet gotten into finger-fucking myself, though I would do a quick dip in to get some more juices to spread around.
I remember being so scared, all I could hear were my wet fingers sliding all around my pussy. That turned me on even more. I came in just a matter of minutes, but stayed exposed like that a few more minutes just for the thrill. That was also the first time I tasted my own pussy juice-I licked each finger clean. I pulled up my panties and used them to wipe off the seat, then pulled up my jeans. I was still buzzing from the orgasm when my mom got back.
While that was the only time I did it in my mom's car, I have most certainly done it many, many times in my own car. And, of course, I'm always a good girl and clean my fingers after a good cum.