Hello Squidley,
I had a similar problem with my 87. I found a used TCI Box that works great. You might want to check out a place called Slicks Great Lakes Salvage. This is where I got my used box from and it has been working great ever since. Maybe I got lucky but that place a good supply of parts. The address is 519 Mason St, Mason, MI 48854. I hope this info helped.
Todd