If you want to see if it's a mini plug for size, see if you can insert a headphone or earbud jack into it.
If it snaps into place, go to Rat Shack and get an 1/8" mono mini plug and solder a wire onto it and you'll be in business. If ya wanna get fancy, crimp a spade lug on the other end.
But first, did you try it without a ground? Sometimes they work fine without it being hooked up and you'll have gone lthrough all this for nothing.