Put a WiFi hotspot in each vehicle, each with a different SSID.
The PDA is only required to detect, not connect with it, then report the SSID it found.
Use the Onion Omega2 (US$5 + P&P), the C.H.I.P. (US$9 + P&P), RasPi 0 + WiFi dongle (<US$10), or there's probably others with WiFi.
Same idea as NFC but much greater range and probably cheaper.
You'd probably need to write a small program for the PDA.
This isn't a "download an app from Google Play and all my problems are solved" sort of thing.
Considering that no information has been given regarding what the PDA/phone/thing is, (as usual), then it is indeed highly unlikely the solution would be this easy