Universal Electronic Car Aerial
Power car antennas typically have 3 wires. One is ground, one is constant +12 and one is switched to +12 or open. Color codes vary.

You need a high side switch to switch the controlled line. 

When the high side switch is active (+12 on that wire) the antenna mast goes up until it hits the upper limit switch. When the controlled wire is low or open, the mast goes down until it hits the lower limit switch (power is supplied through the +12 always-on). 

Edit: Example high-side switch. Note that the MOSFET is a P-channel type. The diode D1 and R1 are to protect the MOSFET gate from transients on the 12V line. Q1 level shifts the input from CMOS/TTL level to 12V. Input high turns Q1 on, pulling the gate of M1 low and turning it on, which energizes the grounded load. D2 is to deal with inductive loads. 







simulate this circuit &ndash; Schematic created using CircuitLab