Its your Sirius satellite, satellite-only antenna. The hole is included in body (roof panel) stamping process.
Sirius terrestrial reception is via the passenger rear quarter window glass grid along with your AM/FM.
When equipped, the GPS antenna would be located under the dashboard, near passenger side airbag. Inside the car, not out.
GPS antenna is only installed with Navigation head unit.
The optional Sirius tuner is required for service to work on your existing head unit. There is an easy way to tell if you have the optional tuner, something like holding the AUX and Preset #1 buttons momentarily with the key on and radio off. If there is the optional tuner, your Sirius ID will appear.
Most likely an aftermarket head unit would require an antenna end adapter/converter and either a built-in or optional stand-alone tuner.
It appears to be a Navigation antenna or Sat radio but my Expy has neither. Stock AM/FM/single CD. The other oddity on this gov fleet vehicle is it has rear seat HVAC controls, stereo controls, and RCA input jacks. They do work so its not like they stuck some random console in there to replace the police one. I'll take a better picture later, my wife took the truck to work (she's tired of driving the little hybrid).
