Carry two cell phones. If I'm kidnapped, the nappers will probably assume I only have one phone, so they'll take one and leave the other. Then, I'll secretly call 911 so the police know I'm in trouble. I'll whisper as many details as I can into the phone, if I can do so secretly. The dispatcher can track my phone using data from the GPS chip and/or the tower the cellphone is connected to. They can also listen to all conversations in the car. This will help them figure out details that I might not have given them such as the how the people in the car sound when they're talking, or other things they wouldn't want the police to know.

I might ask for a bathroom stop on the way to their destination or a waypoint, just so the dispatcher knows where to find me. That can help, because even if they do not pull over, some other driver (preferably a cop) may be able to recall the license plates of the cars that drove by at that time. If they do pull over and let me out, they would basically be driving into a trap, as the dispatcher would send local police to the gas station, where I can then identify my captors to the police, and divulge any information I have regarding the captors that the 911 dispatcher doesn't have.