I'd imagine your oem aerial wiring is still behind the stereo unused (i assume he hasn't smoothed the roof) you wouldn't tap into it or anything (other than power etc) to run a glass mout, it has it's own wiring do just rip it all out.
The previous owner might have installed one for a reason do check the oem connector first as they can break (although you can buy connectors and re-crimp it)
