Jerry, without looking at a set of schematics I have no idea if Ford ever did or does this, but I had a '71 Dodge that used the dome light on a dim glow as an indicator that the tail/brake light power was out - my fuse was blown. It did not go out with speed - glowed all the time, but back then speed inputs were pure mechanical, and it took me quite a while to track it down. You might check your tail/brake light fuse, just for grins. Again, I am not saying Ford had this feature - it's just a suggestion based on my experience with a Dodge, and I had never heard of it before.