And in typical fashion, asking the question caused me to renew my search and I found it!
It’s Dr. Jekyll’s Pepper Elixir.
Which is actually NOT a “real” soda. It’s a Halloween-only version of MacFuddy Pepper Elixir by Orca Brewing in Washington (State). It does say it was popular in Michigan though so that must be where he got it and since it was a seasonal thing, hard to find.
If you haven’t had it, it’s a regular-ish cola, but with a hot pepper finish. Damn good stuff.
Now off to find some MacFuddy…