If you're talking about a left hand drive (American) XK8, it's to the left of the steering wheel, just above the hood release lever. It points almost straight down, so to see it you really need to get on your knees and look up at the bottom of the dashboard. With the door closed you can feel it if you run your hand down along the corner where the dashboard and door come together. Just before you get to the hood release lever you'll feel the OBD port.
Evap Leak
The 1999 Acura TL OBD 2 port is to remove ashtray
The 1999 Mercury Cougar OBD 2 port is under driver side dash left of steering column
The 1999 Mercury Cougar OBD 2 port is under driver side dash left of steering column
The 2000 Jaguar S-Type OBD 2 port is on lower edge of dash; right of steering column
The 1999 Ford Contour OBD 2 port is under driver side dash next to kick panel
The 1999 Toyota Tacoma OBD 2 port is on lower edge of dash; next to console; connector faces down
The 1999 Honda CR-V OBD 2 port is under passenger dash
The 1999 Toyota Avalon OBD 2 port is behind fuse cover
The OBD port is on the right side of the center console-that would be to the left of the glove box.
The 1999 Chevrolet Camaro OBD 2 port is under driver side dash above accelerator pedal
The 1999 Volkswagon Cabrio OBD 2 port is under dash at left side