Prototype Bottom
Jeff writes –

This page contains plans to build a device to interface RS-232 (a laptop computer) to the ISO9141-2 / SAE J1962 (OBD-II) diagnostic connector on many Volkswagen, Audi, Seat, and Skoda automobiles. It should also work on many pre-OBD-II models. It has been tested on my two vehicles, a 1998 VW GTI 8v and a 1998 VW Passat GLS.

Opendiag schematics & PCB layout – Link.