ch changer

My girlfriend owns a 1999 mercury sable v6 3.0 automatic and she has a cd changer she wants in it but it involves hookin it in through the radio antennae but the only problem is is that i looked behind the radio and there is no antennae and i was wondering if n e one could tell me where abouts the cable is in the car and how to go about hooking it up???

The unit in the daash is actually just a Control Unit. The actual Radio/Amplifier unit is located in the trunk on the left hand sife at the rear of the wheel well under the trim. The Antenna is connected there.