The 15 pin amp connector does not plug directly into the i/o board. There are two sets of wires that run off of it and go to two smaller plugs that plug into the bottom (or side if you are vertical) monitor bracket. From there, they snake their way to the io board but it is like spaghetti inside the cab. This is so you can easily pull the monitor out without having to yank wires out of the i/o board.
The thing that I can't figure out is how you are getting power to it right now.