A data cable can be made by purchasing a Serial Cable and a plug from the Purple Computing Project. I bought two plugs and made two different cables. I made the first cable by cutting off one end of a Serial Cable extension and wiring in the 'purple project' Garmin plug. The other cable, I made from a 'gold' series headphone cable and a serial adapter from Radio Shack combined with the 'purple project' Garmin plug.

The GPS unit is connected to the serial port (9 pin (DB-9) or 25 pin (DB-25))
of the computer by using the cable described in Garmin's User Manual. 

There are only three pins used by this program-Data in, Data out and Ground.
                                                           Computer 
        Garmin GPS*                           9-pin serial     25-pin serial

        Ground          pin 2   ------------->  pin 5   Ground    pin 7
        Data Output     pin 4   ------------->  pin 2   Receive   pin 3
        Data Input      pin 5   ------------->  pin 3   Transmit  pin 2

* The pin numbering is valid for the Garmin GPS 45. 
  Consult your manual for other models.