That is a DC plug.
Now the most likely is that it is a straight 12v from the battery.
However it could be connected to anything, it might be 3v, 4.5v, 6v, 9v, the previous owner could have put a universal adapter back there.
Best thing is to put a Multimeter/Voltmeter and find out what voltage is coming through, trouble is it may give you a false reading (higher) without a load on it.
Personally I would either trace it back to it's source, or if that is too much trouble, just tape it up and tuck it away somewhere.
What I wouldn't do is just plug random things into it to see what happens.