Hi Jeff from across the county line!
I have a buddy who is really into the early Ford scene, and based on that, my recollection of a rat rod is best summarized by the typical jalopy that a teenager in the '50's would throw together based on a 1920's-early 1940's Ford. You'll find money spent on parts to make the typical flathead V8 (or other engine) go faster, but little money spent elsewhere.
Primer or flat paint jobs (maybe even painted with a brush) are the norm, and the upholstery may just be one of those old wool "New Mexico" type blankets thrown over the old torn seat. You would never find anything fiberglass, billet, or with power assist on a rat rod. The rat rod is all about fun and being able to drive your ride without worrying about rain or a paint scratch to dampen the sheer joy of running something you put together.
So, a 1970's Opel mirror on a rat rod? Well, maybe the simple lines of the round metal door mirror would work, but an Opel rear view mirror? Nah, the roof mount and black plastic wouldn't fit it the rat rod style IMHO.
73 Manta (safely in the garage, away from the snow, and anxiously awaiting spring!)