Sounds like they may have done some shoddy work...

Check your wires and keep speaker leads away from power leads, never criss-cross them, etc. Make sure the ground is good (which it sounds like you did). You'll get induction noise.

Finally, invest in a filter to go in between the power wire for the amp and the battery. This should eliminate any noise picked up from there.

Double-check all your connections, use good quality materials, etc. and blah blah blah...