There should be one there. There's some chance someone was cheap and just ran the wire out through a hole in the siding.

One of the biggest thing I've learned, in working with old houses especially, is that you can't "count on" ANYTHING being done right!

Yes, there SHOULD be a junction box behind the light. Will there be? Who knows?

pharkus, one thing I've learned from THIS house is don't count on anything being done right!! (early-80s, passive solar, experimental design, owners running out of money as it's built = sketchy) The motto here is, "Code? What code? Morse? Zip? Area?"

I found an alternative that looks promising -- the Stonco Nytewatch (http://starrynightlights.com/blog/2008/10/10/stonco-nytewatch-motion-controlled-security-lighting/) -- that can use either a regular junction box or a flat, round plate that I imagine would be a lot easier to retrofit behind the siding.

You'll have to take it off to see what you have, but another option *might* be a pancake box on top of the siding, although you could run into wirefill problems/limits.

OK, I removed the fixture and looked inside with a flashlight as well as I could. Here's what I found, going from outside to inside.

1. FIxture attached to a 5" square vinyl siding panel that must be intended for outdoor lights.
2. The 2 wood screws holding the fixture were screwed into the vinyl itself, nothing more.
3. The empty space in the wall, full of wiring.
4. A type of plain, plastic junction box that doesn't seem to have any holes for attaching or threading things. I think the wires were just sort of guided around the outside of it and stuffed inside. But without 1/2' holes, the RAB motion detector, for example, would have nothing to attach to.

I'm guessing I'd need to:
1. Pry off that vinyl panel from the siding.
2. Undo all the wiring.
3. See what (if anything) the junction box is attached to, and replace it with the kind that has 1/2" threads.

Of course, if that plastic box is just held in place by the wires themselves, and I couldn't tell for sure because it was so crowded in there, I've got myself a big problem trying to install a new one!

That "junction box" sounds like a "plaster ring" to me.

Anyone else?

What's a plaster ring?

I found some photos of plaster rings, but they were all flat and they don't look like this one. This looks more like a round junction box with plain surfaces, maybe 4 inches diameter and 1.5 inches deep. No holes, no projections. The color is yellowish, maybe from age.