Check your amp's ground AS WELL AS your head units ground (alot of people overlook this, but 25% of the time it is the problem) and check to make sure your rca connections are tight (on the amp as well as connected to the head unit). Also, wrapping wires in tin foil would probably just induce more noise...

Also, if your grounds are ok then you can run your power wire, speaker wires, rca wires, all intertwined with eachother and have 0 noise.