For a wiring diagram of any type of car, you can try image libraries, wiring diagram sites, or specific Porsche user sites.
Wikimedia Commons provides free public domain images which you can use however you like, whereas if you take a picture from Google Images, which has a wider range, it's best if it's for personal use only.
As for wiring diagram sites, you could try pelicanparts.com, which offers 17 downloadable pages of wiring diagrams for the Porsche 924, as well as the Porsche 924. morediagram.com has a PDF of the wiring diagram, while tune-town.com offers a wide range of wiring diagrams for Porsche car stereos.
If it's a question of a missing manual, you might find one at calameo.com, otherwise you can head for the Porsche owner sites, community forums where you can post a question regarding wiring diagrams or any aspect of Porsche 924 ownership.
Go to pca.org, home of the Porsche owner's club of America, or even porsche924.co.uk if the American site doesn't have what you are looking for.
For a hand in reading a Porsche wiring diagram, let this YouTube user show how it's done: