Around about 2000 something important happened. The various standards organisations around the world introduced standard for communicating with car computers. Prior to this cars had been using computerised systems but all required their own specialised service equipment. The introduction of standards meant that the same service system could be used on a BMW as a Ford or Peugeot etc.
The standards have been phased in over a period of time so it depends on a cars age (manufactuter, country...) as to whether it has the standardised facilities. In europe, for petrol cars, it was around 2000. For diesel the standards appeared later in 2004-ish.
One part of the standards was where the socket to connect to the car should be sited. This is within 1m of the driver. On one of my cars its in the footwell. On another its in the centre console (hidden but easily accessible under the ashtray unit).
The socket is the OBDII interface and looks like a SCART socket found on most televisions.
