These were used on GM full sized trucks and were meant to handle the full alternator output as well as provide battery power to everything but the starter, so they can handle a good amount of current. They’re great for use on projects where you have a lot of accessories that require power or...
I got mine from a couple late 80s - mid 90s chevy trucks. They are located at the top of the firewall under a plastic cover that runs along the passenger side. Works very well and is about the same as the one posted up top.