Just speaking from experience. I have had the TBL burn out and the CC stopped working. As have many others on here. I don't the exactly how its wired all I know is from experience.

 

I have had 3 Ford trucks....2 Expys and a F 150 ...CC still worked even though bulbs were burnt out on all three


big difference between bulbs being burnt out and not getting power to the light assembly though

 

Only the 04+ F150's had CC tied to the TBL. Not sure why your CC works without the TBL working, but there seems to be some issue where your truck still thinks it works.

Have you owned this truck since new?

Your Expiditions and other F150's wouldn't count (unless they're 04-08 F150s).

Just search google for "cruise control third brake light" and you will see many, many posts about how cruise is inop if the TBL burns out or if you try to add an led without a resistor.

However, all of that is besides the point. First test to see if a new bulb works. If not, it's obviously another issue.