That's not what he's interested in. He doesn't need to adapt the stator signal (the "W" terminal signal) to a tach - he's got VW stuff that is designed to use the stator signal, no adaptor necessary.
He needs to connect his "W" connector to the alternator's stator terminal. According to some discussion of the DSL-1 over at the Infopop biodiesel forum, the "P" terminal is the GM equivalent to the "W" terminal.
FWIW, the wrong terminal can have quite... ah... hilarious consequences. Don't plug the "W" connector into the "W+" connector, or the horn will honk when you're cranking
(Note: this is on a VW alternator. Not a GM alternator.)