help with non bose stereo

i purchased a million g2217a stereo today, after unplugging and removing the factory deck i notice that there are 3 cords going into the back the main plug, the one to the small screen at the top and i have no idea what the third one is for ? can anyone tell me what its for ?

Wondered the same when I replaced mine. It's now unplugged and has had no effect on anything.
The only thing I can think of is if its got something to do with the automatic volume thing that turns it up as you go faster.