1) if you switch speaker wires back and forth and both sets of speakers will play then it's not the wire or speakers, move upstream
2) if you swap right and left RCA coming into amp and you get sounds from both speakers, then it's the RCA cables, replace
3) if you swap them and you still only get sound from same side then it could be bad amp or it could be upstream still. Since you already changed amp, that is unlikely.
4) Repeat with all cables gradually moving upstream until you get to the HU.
You did make sure somehow the balance wasn't set to just one side, right?