So, you really have two options here.
Option 1: Make out and don’t worry about the driver. Sorry, but you’re not going to be able to put moves on anyone without his seeing. What you can do, though, is make out in a romantic way—not a dry-humping way—so your cabby knows there’s no actual funny business going on. He probably doesn’t care about a light make-out session. He probably does care about body fluids or illicit sex acts going on in his car.
Option 2: Don’t make out, but go for some light, over-the-pants touching. Because the seat’s in the way, the cab driver won’t be able to see what’s happening below your waist. Sit next to each other and rub her inner thigh, creeping up slowly between her legs—it’ll be a nice tease for when you get home. In short, think of a cab as an OK place for some light foreplay, not as an OK place for sex.