I'm the April fool: Why can't I turn off the engine in my 240 D?

The most of dash lights in my recently acquired 240 D did not work, so today I replaced the bulbs in the speedometer cluster/instrument panel. It seemed like an easy repair, and did not take long to finish.

However, when I put everything back together, the glowplug light did not come on. When I started the car, it ran fine, but when I tried to kill the engine, it kept running. I could even remove the key, but the engine idled perfectly.

When I took the instrument panel back out, there seemed to be the odor of burnt electrical wires from the general vicinity of the clock, but I can see no evidence of shorting.

I’m reading my Haynes book, and it mentions the fuel cut-off solenoid is a likely suspect. Is this likely, and could its failure be caused by something I did while removing / replacing the speedometer cluster/instrument panel?

Can anyone tell me what I did wrong, and better yet, how to solve this problem?