I recently bought a 94 Explorer, and it had a black plastic disk attached with double-sided tape to the dashboard, just to the left of the glove box. It's 3.5 inches in diameter, 1/8 inch thick, and the only marking is "S3" on the back. Maybe it's an RFID, like for automatic toll payment? Some kind of anti-theft tracking device? Spyware? Just a decoration?

 

Something very similar to this came with a GPS unit my parents just purchased. In this instance you stick the flat disk to the dash or another surface so that you can then use the suction cup on the GPS to "securely" mount it.

 

I got a Garmin GPS system an it comes with a black plastic disk to mount to the dash like MCDavis said. Do you have a picture of this black plastic disk?

 

The Nextar S3 gps uses a mounting disc as you described...