2007 yaris sedan third plug on wiring harness

[Frankieo]

I'm installing a after market dvd player into my car but there is a third plug going into the stock head unit and I have no idea whats it for I have all the wires hooked up to my new unit but don't know why there is more connectors. I took for a drive and got a check engine light on took it too autozon re and they gave me a code of p0500. Took the car to the dealer and they said that the old radio needs to be connected because that plug sends a signal to the car computer and let's it know that the car is moving. Has anyone came across this problem or know of way to bypass this?