Just to tie up the loose end, I pulled out the old radio. There is a male blade connector at the bottom on the back of the radio. The goofy aluminum bracket looks like it would end up right along the bottom of the radio if the spare wire were connected in that spot.

Thanks for all the helpful replies!

Tom

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2016 E350 4matic wagon
2010 ML350 Bluetec
1995 E300D Gone, but not forgotten.