I've frequently mentioned to my wife that I'd love to see her drive either topless or naked. She's mentioned that she's sort of interested, but also scared. During one business trip, she mentioned she'd be willing to do it because we were driving a van, thus higher up than most vehicles and with more coverage. So on the trip back, with her at the wheel and dusk approaching, I ask her to pull down her top. She smiles, saying "I wondered if you were going to ask" and starts pulling down the straps of her tanktop.
As she readjusts herself, her clothes and the seatbelt, she says "I must really love you...", with a smirk. Finally she's driving, blushing, breasts exposed, and I take a couple pictures for good measure. She notices that I can't take my eyes off her, which is the reason that I asked to do this while she's driving, not while I'm driving. I'm noticing that, despite it being plenty warm, her nipples are hard the entire time. After a good half hour, she pulls her top back up again, complaining that the seatbelt was digging into her chest. Not that it matters, the sunlight was almost completely gone. When we stop for gas, I asked her how she liked it and she replied, grinning, that it was kinda fun and we'd have to do it again.
We have had a few more circumstances of her driving topless, but she's got her rules. She'll only do it once the sun has dropped below the horizon, which is a very limited time of useful light before it's too dark to see or photograph anything. Also, it's tough to do if she has anything other than a tanktop on. But you take what you can get and she's even annoyed if we go too long between sessions. Time to start pushing the limits if she'll do some earlier in the evening.