The antenna switch is not particularly necessary, but it does add to the fun to extra switches in the dash. The dealer price I was quoted a few weeks ago was around $60US. I found one in the junkyard, but someone had ripped the back of it off and the internals were gone, so they left it. After seeing the dealer price I attempted to repair it, which turned out well and was actually much easier than I had expected, given my failed repair of earlier switches. It helps to grab a couple spare defrost (early) or hazard switches at the junkyard to have spare parts and an idea how to put them back together.
Now I've got to figure out where to put it, I'm one switch location short at the moment. Hope you don't have that problem.
Switch works exactly opposite of the other dash switches, it interrupts the power wire from radio to antenna when depressed. There is a harness that goes with the switch, but it is very simple, two wires, three connectors. One end plugs into the switch, the two other ends are single wire connectors with the plastic covers, and they connect to the radio (ant rem +) and the factory antenna power wire. This is located near the rear of the radio or else somewhere in the driver's side of the center console. Easy to find because it is attached to the antenna wire itself, the wire you want is the smaller diameter wire. You can see this pretty easily in the trunk before you go hunting inside the car.
If your stereo shop ran their own wires then it is very unlikely that they messed with this, and it is still there. Many people and shops use it to run the power feed for an amp in the back if they are too lazy to run their own.
The switch harness ran around the heater vent outlets and then down to the radio on the car I removed it from. Lots of extra wire. I plan to mount the switch in the Bertone parking brake console, and it seems there is plenty of wire in the stock harness for that too.
Please let me know if you want pictures of the harness or switch or anything. I haven't installed it yet, so a pic would be pretty easy.
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1981 242 GLTi 1967.5 122S
Seattle, WA

The 850 had an antenna switch to interrupt the power option when the cd player or tape deck was used. Its an easy number to scribe a 240 switch coverplate to take the 850 switch or just use any on off switch (IE: The old "rear window" switch that is now on the drivers armrest use to be in the switch panel).

The switch should not be necessary if your radio has the remote amp turn on/power antenna output. My '82 240 has the switch, but it only switches keyed power to the power antenna trigger lead. The stock radio in my '82 lacks the power antenna output. I also have an '86 240 - no power antenna switch in the dash, but the stock radio does have an output for the power antenna.

I prefer the setup in the '86...

-Matt