I'm guessing I'll find the problem somewhere near here...

Looks likebthe rear window washer fluid reservoir has been leaking for a bit. I'll change the tail light gaskets while I'm at it.

The tail light wiring looks good, I haven't found where the ground wire attaches to the frame yet.

Yuck!

Yeah. :ack: I'd go to town on that area and get a new bracket and hardware.

Yuck. Here are the parts diagrams with relevant part numbers:

https://www.glaver.org/transient/battery1.png

https://www.glaver.org/transient/battery2.png

While you have it apart, I'd wire-wheel the rusted body parts, prime, and then either re-paint with body color or some more durable coating. If there are any thru holes, make sure they are cleaned out thouroughly and on the underside, and similarly primed and painted.

Bag the positive battery terminal and fold it well out of the way - you don't want the explosive disconnect in there going off from sparks or static.

In anything other than a wagon, this is pretty easy. Since the wagon has full trim back there, it gets a lot more inconvenient.

Make sure the battery vent hose is not plugged and securely reconnected to the battery when you're done.