Main fuse box question...

I apologize for my ignorance on this subjuct, but I'm trying to figure out what the fuses are plugged into are called. The fuse plugs into them, and so does the wiring harness on the backside. I took my 2016 Tundra into the dealership for flashing lights on the 4hi and lo, abs, brake and check engine light. They told me that the first step is replacing the trailer brake relay and then the entire main fuse box, the explaination was that if the trailer relay is bad and I replace the fuse box first the "bad" trailer brake relay would cause the fuse box to go bad. I generally consider myself a fairly technical person and this didnt sound right and they gave me the wrong part numbers which is why I am opting to take a swing at this repair myself. Any help or guidance would be greatly appreciated.

 

There s/b a CEL on dash - has it been read? Even if not lit - there maybe a pending code for a clue.

 

Ill definitely let you know the codes when I get off of work, I think there was 4 or 5 codes that were showing