TLDR: I've spanked seven different women in the backseats of five different vehicles over the past 23 years.
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Mazda 626
The first time I gave a backseat spanking, it was in my Mazda 626. My girlfriend and I had already experimented with spanking, but when you're in high school, you have to make do with what you can. I distinctly remember parking at a park near her house and turning her backside bright-red. I won't go into detail beyond that because of our ages, but it did happen more than once.
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