It should be connected to one of the HDMI ports on the TV. If the trailer came with a DVD player it's probably plugged into HDMI 1, and the connector on the wall plate is probably HDMI 2.
Assuming you've gone into the TV menu to switch inputs between the available HDMI ports, the next step would be to remove the wall plate to verify that a cable was installed on the backside. If it's there, then you would have to look at the back of the TV to see if the cable is plugged in there.