General Installation
On most vehicles the OBDII port will be in the driver's side footwell area, possibly near the steering column or fuse box.
BMW
On some BMW's the OBDII port may have a plastic cover that has the appearance of a different connector.
Mercedes Benz
On some Mercedes' the OBDII port may be located behind a plastic cap labeled "OBDII" or "DLC".
Volkswagen
On some late 90's/early 2000's VW's the OBDII port may be located behind a panel/door in the ash tray compartment or beneath the HVAC controls.