B2909 Belt Tension Sensor Fault indicates that there's a problem with the front passenger belt tension sensor. It's used in conjunction with the occupant classification sensor (pressure sensor) to detect the presence of a child in the front seat, which would disable the passenger side airbag in a collision. An occupant classification sensor fault will also flash a 16 airbag code, but with a DTC of B2290. Shorts or open circuits to the belt tension sensor will trip the 16/B2909 code. Try disconnecting the battery, wait several minutes and re-seat the electrical connectors under the passenger seat and see if the problem is resolved.
As mentioned by another user, the yellow plug is for the optional side air bag. If the side of your seats don't say air bag, then don't worry about that connector.
If the airbag light started flashing immediately after installing the replacement ICP, try reinstalling the old ICP and see if the airbag light stops flashing. If it does, there is probably something wrong with the replacement ICP.
More information
here (PDF). Troubleshooting/Pinpoint test procedures are for the 2009 Mustang and should not be performed on other vehicles.
