I've had my D3 and my Pioneer GEXP920XM unit for about a month now. I feed the XM unit with a separate power cable connected directly to the battery. That power cable has a 25amp in-line fuse. The XM unit also came with a 2amp in-line fuse.

All was working good for a month and then all of a sudden my XM stopped working. I found out that my 25amp fuse was blown. The little 2 amp fuse was fine. I replaced the 25amp fuse and that blew within minutes. I then replaced that with a 30amp fuse and that blew as well. The 2 amp fuse remains fully intact.

What the hell is causing this? And why would the 25/30 amp fuses blow before the 2 amp fuse?

because there's a short in the wiring between the 2 fuses. More than likely the wiring is pinched somewhere.