I am trying to fix the electrical in the 68 FB 400 convertible we just bought a month ago. I have traced out all the components in the engine using the wiring diagram, except for one. I have a very small relay or switch (can't really tell) mounted on the firewall above & to the right of the master cylinder. It has one wire attached even though there are two posts, and it runs through the firewall. Anyone got a clue as to what it is used for? I tried to post a picture, but could not embed image into this post.

Power convert top breaker?

Fig 12-50 in:
http://www.thefirstgensite.com/library/68sm/chaselec/chaselec.html

X2. Power top relay

It is a self-resetting circuit breaker, not a relay. For power tops and/or power windows (30A).