Jon.
The first thing to emphasise is to not take the error code literally. Cheap code readers are known to give misleading results, so unless yours is a professional quality reader it can not be relied upon. It is not a field I am knowledgeable about so I suggest you wait for a more informed response here or take the car to a garage that has a good auto electrician.
The second point to make is to not replace parts at random like your garage has done. Just because another person solved a similar sounding problem with a new brake light switch does not mean his solution will work for you. You might strike lucky, but more likely is that you will throw good money after bad.