In the trunk on the drivers side, tucked into the quarter panel, usually behind the little door that holds the first-aid kit is a connector that has a harness leading up to the back of the radio. If your car has the trunk-mounted CD changer, this is where the changer gets plugged in. If your car has no CD changer, the harness and connector will still be there.
This harness will not effect how the car runs.
Under the car, buried in that area of the quarter panel is the EVAP system Leak Detection Pump. If those parts got damaged in the collision, you may have a vacuum leak. This could potentially cause the car to run rough.
Get the car scanned for trouble codes. Inspect the LDP and hoses for any damage.