D150 wiring issues, help!

Yup, the ammeter in those plastic housings can be a problem. Many of the housings I've removed over the years were melted around the ammeter posts due to overheating from one reason or another. Mostly loose connections, but, sometimes it was because of too much draw on the system due to added accessories, or, too high powered an alternator installed. Whenever I have those clusters apart, I always solder the threaded post to the buss bar, then make sure the connections are clean and tight.