1990 Ford F-250 What exactly is a starter relay switch?

Hi Guys. I was having trouble with my truck not starting so we replaced the battery, voltage regulator, starter & alternator before we discovered it was just a positive battery cable. While we were having the problem, I disconnected the trailer brake connection from the battery to elimanate that having a possible short & causing the problem. Last week we took the truck to the repair shop to have the trailer break connection reattatched & the system checked so we could pull a trailer. We were told our starter relay switch had a broken post & that is what powers the trailer breaks. Is a starter relay switch the same thing as a voltage regulator? I sure hope not because we only paid about $15.00 when we changed ours ourself. Although we paid over $200.00 to have it repaired. It seems to me that if the starter relay switch had a broken post you would be having some problems, wouldn't you? Thank you.