I think the way to solve this mystery is to figure out what the combo relay and the turn signal have in common.

I'm not looking at a wiring diagram, but I kinda doubt there's any commonality in the electrical portions of the wiper and signals. Sometimes really odd behaviors happen when a ground point goes bad (corrosion blocks it from conducting electricity) and the current seeks another path to ground. It's worth checking the manual to see if the wiper relay and the signal flasher share a ground point, but I'm betting the answer is no.

Both of these functions are activated by the same stalk. I wonder if there's a broken wire or damage to the wiring insulation inside the stalk or somewhere in the steering column? That's where I'd look first.

- JimY