Yeah, find the wire that controls the aerial (my car was yellow/black although I think sometimes its blue), probably be one that is not connected to anything at the moment. Or find a wiring diagram for your car that shows the colour of the aerial power wire.
Most of the newer headunits don't have a separate aerial output, so usually you just connect the 'remote' line (the one that controls an amp) to the aerial. However this will make the aerial rise every time the car is on.
So what I did was buy a switch from jaycar and just put the switch between 'remote' and the aerial wire. All the aerial power wire needs is a constant positive voltage to be in the up position. Then I just push the switch that has replaced one of my coin holders when I want to listen to the radio.