My current setup is sony HU, two alphasonik amps, 10" alphasonik sub and kappa perfect component speakers. So, I decided to install my new Alpine iDA-x001 today. Cut off the old harness and soldered the new harness. Ready to go. Plugged the harness and RCA cables from the amps into the new HU and kaboom! I hear static noise and see smoke from behind the HU. The fuse was blown on the 12V line and HU was dead.

So, me and my friend inspected the soldered wires and make sure everything is connected fine. We concluded that it was the defective HU. Then, we get a new fuse and hook up my friends HU only to the harness and it turned on and functioned fine. At this time we weren't sure which RCA pair was from the sub amp and speaker amp. So, we first only plugged a pair to the sub RCA jack and sub played fine. So, we plugged the other pair of RCA into the front speaker jack but nothing from the speakers. Then, we switched the RCA pairs on the jacks and I hear short static noise again followed by smell of smoke. Another fried HU.

What could be wrong? Could inserting sub amp RCA into front speaker jack really fry the HU like that?

 

Don't know but it sounds like you sent power to the HU instead of the speakers. Re-check your wiring. HU out to amp in, amp out to speakers?
There has to be a mistake in one or more of your splices.

 

Or, you have a seriously defective amp that is sending speaker rail voltage back up the input line.