Do you want to wire your car so that the Accessory circuit stays on until you open the door after you have turned off the ignition, like many modern cars? Built your own remote start system? Control headlights with a latching relay?
This page at The 12 Volt explains how to do that with standard 5-pin 12-volt relays. The entire site is dedicated to similar "tricks of the trade".
At Hotrodders.com, this page provides an overview of automotive uses of relays. This page also offers links to other useful pages, including headlight relays, assorted car wiring diagrams, auto wiring basics and a fix for power windows that use those stupid "wiring eliminators" in the door jambs.
Here, mechanical latching relays are explained.
GM-style power window switches are Dorman 901-018 or 49243 and the special pigtail is ACDelco PT185. A wiring diagram is found here.
A 12-volt latching relay is here.
Neutral Safety switch operation, wiring and installation information.