Here is the AC power jack that is used in the Empeg/Rio Car players.
I have cut away the upper portion of plastic from it to reveal the innards.

Note the three solder pins (electrical connection points) on the bottom.

• the left pin is ground (-).
  
• the centre pin is for the "sensor".
  
• the right pin is 12V (+).
  

The cut-away portion of this photo shows the centre post (+),
and a spring contact (-) beneath it.
There is a third internal contact (not visible here) for the "sensor".

With no plug inserted, the the spring (-) touches the sensor contact,
so an ohm meter should show zero ohms between the two leftmost pins.

With the AC plug inserted, the two contacts do not touch each other,
and an ohm meter should show a very large resistance between them.