Think I blew my HU amp.

[DJ_SpaRky]

Speakers are a closed circuit with the HU or Amp. I doubt it has a "Safety Mode", it's probably shot, but try hooking up an Amp (cheap one from Walmart will work and you can take it back) to see if you get sound from that.

BTW, what make/model HU?

[Excelsior]

Modern, high-power head units do NOT work with grounded speaker wires! With some old, low-power set-ups, the instructions would say to ground the (-) terminal of the speakers and just run the (+) wires directly from the radio to the speakers, but this does NOT work with most modern radios!

The good news is that most newer radios do, indeed, have a protection mode that shuts down sound to all speakers if any one speaker wire gets grounded. This is probably why you lost all the sound when you installed the second speaker -- a wire to that speaker is probably grounding out. Now, it's still possible that you blew the head unit, but the easiest thing to do would be to run both the (+) and (-) speaker wires directly to the speakers, and make sure no bare speaker wires or terminals are touching metal (ground). If you still don't get any sound, then you may have a bad head unit.

One other thing. You really don't want to use a test light on any car that has computer circuitry in it. One bad "probing" of that test light can fry a computer!