I have the exact same issue now on my '89 'winter driver' Reatta.... And I don't want to 'borrow' the switch assembly from the good '89... Unless I find either a cheap working used switch assembly, or a tutorial here on fixing one, I may have to try to make my own tutorial (if I'm successful)... Saw one on eBay, asking $50; I hope there are ones out there cheaper than that.
I also have an issue with both high-beams not working; but bulbs test fine, and I heard the relay working (and even swapped 2 other relays in), so I'm inclined to believe that it's a wire or connector between the relay and the headlamps themselves. I've glanced at what I believe is a circuit schematic, and unless I'm mistaken, I do not think the dash switch has anything to do with high-beam problem (since I have low-beams, and the relay switches from low-beam to no lights at all in the high-beam setting (with the blue dash indicator light coming on)).