Originally Posted by Ron in BC
2. The fuses are called by Ford "relay fuses". They don't look like conventional fuses, more like little boxes and I believe that they're called Type M fuses.
The term "relay fuse" makes no sense - it's a relay, or it's a fuse. It could be a fuse (perhaps in the standardized style of package normally used for relays), or it could be a circuit breaker, or it could just be a relay. How many terminals did it have, and what did it say on it?
The Ford F-150 manual for 2018 (the one I have handy) has one table listing both fuses and relays in the panel. #43 says it protects
(not "is") the "Trailer tow lamps module", with a rating of 20 A, and the footnote for the capacity says that it would be an "M case fuse." That makes it a simple fuse (although a time-delay type) - nothing to do with a relay - in a cartridge format (so it is a plastic box with two female sockets on the bottom, rather than the more common ATO or ATC plastic block with two male blades on the bottom). That manual even has a drawing to show what it looks like. One brand is the Littelfuse MCASE
"M case", "Type M", and "Class M" are all terms used to describe fuses or relays, and are very different.
Just as restarting most computer systems actually makes a difference (the support people don't tell you to do this just to waste your time), removing and reinserting a device might fix (for now) a marginal terminal connection, such as one suffering from corrosion, especially with a wiping-style contact.