rodbailey

The computer cable:
1 female serial port plug, 1 2.5mm male stereo plug. Looking at the serial port plug from the soldering side, the wider side up, you connect the first top pin (counting from the right) to the base (ground), the third top pin to the middle and the forth one to the tip of the stereo plug. That's all.
GPS - Cartesio adapter for use with your GPS - computer cable:
1 male serial port connector, 1 2.5mm male stereo plug. The same as above, you just count the pins from the left: the first to the ground, third to the tip and the fourth to the middle.
An adapter from 1.5 to 2.5 mm may be a solution for you as well.
For those who have Cartesio II, Garmin GPS and you want to make a direct cable (without utilizing the computer cable:
Pins on stereo plug:
tip - data out from the instrument (Garmin white, first L from the notch on the round plug)
middle - data in the instrument (Garmin brown, second R from the notch)
base - ground (Garmin black, second L from the notch)