Ignition switch or wiring harness?? Way Weird!!

[mr2dux]

Okay, so I come out to my 92 300sd after work yesterday, key in the ignition, wait for the glow plugs, and then turn the key to get my favorite car to rumble into life......but not today. Turning the key does not start the engine. It makes the horn honk!! Has been working perfectly until this moment. And the abuse I took at the office today was intolerable e.g. "Hey, if you need help just honk!" Things degraded from there....
I was able to jump it by connecting a wire across the terminals of the starter to get the car started, and now have a flashlight, and a dedicated piece of wire in the glove box, so I'm not stranded, but with that nice a car it is really bad form to have to jump start it every time.
Anyway, what could possibly be wrong? and more importantly, how do I fix it?? Is it a failed ignition switch? short in the wiring harness? failed ground that causes all sorts of weirdness?
Any thoughts and ideas are welcome!! Thanks so much!
Mark