OEM Radio works w/sound, Aftermarket works... without?
 

Before I begin this, I'll get a couple things out of the way so as to rule out the obvious potential problems (Also a TL;DR):

• Deck is fully functional; was sent in for repair and worked with no errors
• Aftermarket radio wiring is correct; tested it in another Subaru with no errors
• I have tried another aftermarket unit with the same results -- no sound
• The unit was working for a good month before problems arised
• OEM radio works with sound

I have a Pioneer AVH-X4500BT head unit that I received over the holidays. It worked fine for the first month I owned it.

One day, I noticed that the sound would be intermittent, switching from static to music, and then nothing. If the aftermarket unit is left on without sound, the internal amp's protection mode kicks in and shuts the amp off, displaying "Amp Error". Oddly enough, whenever this would happen, a quick car jolt to the right would fix it temporarily.

I did some searching and found out that this only happens if there is incorrect speaker wiring. So, I figured it was my crimp wiring, and I decided to redo my wiring harness, this time, soldering everything. To my dismay, it still did not work. I figured it might be a faulty harness, so I tried it in a friend's car, and it worked with no fault.

The kicker to this story is that when I put in my OEM Subie radio, I get sound.
I measured the voltage, and I get 12v at the power, and 5.5v at each speaker wire, even when it's connected to the aftermarket unit.

So before I take it to a professional, (Soundsgood, I'll be bothering you soon) what could cause this to happen? Why would the OEM radio emit sound, but any aftermarket units do not?

What year and model is your Subaru again?
Not all deck circuitry is the same. If you have a short in one of the speaker's wires then the OEM deck may treat that short differently (as in it may not go into protection the same way the Pioneer will). We will get a speaker tester or tone generator and see if the speaker plays a tone with one test lead sent to ground and one sent to positive. We will try that on each speaker until we find out which one it is. Once we find out, we'll go directly to the speaker and check the connections there. Repair or replace some wiring and you'll be good to go.

Correct diagnostic equipment is extremely useful for stuff like this and that is the reason why we charge what we charge. :)

It's a 98 Impreza. Looks like I'll have to pay you a visit this week!

Another easy way that sometimes works is:

Roll up all windows.. turn key to accessory and turn the stereo on.. sometimes you may hear a quiet pop from the speaker that is shorting to ground. Find speaker, remove short and voila.

If you have torsion bars for the trunk lid and have installed aftermarket speakers then I'd start there.
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Update:

It turns out that one of the rear speakers was shorting out. One of the wiring terminals snapped off, causing the short.

Considering that these speakers are at least a decade old, it's time to upgrade!