Upgrading Alternator

You're on the right path

Either the wiper motor is drawing a hell of a lot of current (which I would think would blow the fuse) or else the accessory side of the ignition switch has a drop

The basic current path is from the battery, through the firewall, to the ammeter circuit, to the ignition switch, and to the accessory buss on the fuse panel.

With what you've measured so far, I'd try to get a clip lead up on one or the other of the ammeter terminals and measure the voltage there with wipers on/ and off

Better yet, yank out the ignition switch and measure the RED and the BLACK with wipers on and off