85 has no tach amp. The black round thing in 85s is just an empty socket. Get a black thing from a pre-85/mid-84 and maybe also get another tach to go with it (the pins are different but it might still be usable).

Or, a totally different technique, hooking into the pickup that's already there:
http://www.peachparts.com/shopforum/showthread.php?t=198203