So guess what I found behind the dryer? Yeah. A big hole (this was after the old newspapers and chicken bones). There was once a time when things shocked me, but that time has passed. It really makes dealing with things much easier. This huge void goes way beyond the standard size required for venting in any country, and it completely explains the evidence of vermin we were seeing. But, horror and chagrin aside, it did give a full view of the plumbing situation.
And as you can see, we have a variety of electrical and pipe, one of which looks like it was picked up at a rummage sale at the Hoover Dam. You can also see one of the many random pilot holes (lower left) that perforated the wall. I am not sure if the goal that inspired the holes was ever achieved, but they were humorously plugged with paper towels and old newspaper and then sealed with numerous coats of paint. It is situations like these that you thank your General Contractor heartily when you discover he always buys extra dry wall and mud.