So I got those accessories working by cleaning the contacts on the electrical portion of the ignition switch. No problems there. Oddly, the cruise control just started working again of its own accord, so I'll take that one as a win.

Now, I'm getting a very strange vibration when decelerating at speed. It's very prominent and worrisome. There's something very much not right. As long as I am accelerating or maintaining speed it does not occur, it's only when the car is slowing down. It's a manual transmission, and the vibration does reduce somewhat if I leave it in neutral while I slow down, but it's still there. Any speed less than 20-25 mph and the vibration stops completely. I might compare it to the vibration you'd get when putting it into 5th gear instead of 3rd by mistake, except louder.

It started during a fairly heavy rain and I haven't driven it yet in dry conditions. I haven't had a chance to make a thorough visual inspection, but I couldn't see anything amiss when I took a quick look around. From the driver's seat position it seems to coming from the front right of the engine bay. A passenger in the rear said they thought it sounded like it was coming from the rear. Needless to say I don't know where to start. I was thinking possibly engine mount wear?