so, just for closure on the post. I re-flowed, soldered two wires on the worst offending corrosion, and re-installed. So far, there has been no repeat of the problem, but as it's an intermittent issue anyway, I won't feel comfortable saying it's solid for 500+ miles. Also, I went ahead and bought that Jbox for sale elsewhere on the forum and plan on making this jury-rigged one my spare backup emergency box.
Thanks for all the help!