Ammeter NEVER shows discharge

The factory ammeter measures all current flow in & out of the battery w/ a few exceptions. It doesn't measure transient under-hood loads - starter, horn, nor factory in-cabin options like spotlight or (I think) convertible top motor. Also, it wouldn't measure an add-on stereo which tapped the big BAT wire (red) before the ammeter.

Thus, as you wired it the same, you should see discharge w/ the headlamps on high-beam, but it won't be much say 5 A on a ~100 A scale, so you might have to look close to see the needle budge. Try actuating the brake lights at the same time. I have a neat clamp-on DC ammeter for easily measuring currents in various wires.