Car audio problem:?

Your ground or power cable could and probably are the problem if the RCAs are good. If they are making contact, but not good contact not enough power will be able to flow and it will make the sub quieter. The power steering would have nothing to do with it because it does not get its power from the battery. The power steering pump is run by a belt off the crankshaft so it is mechanical power driving it. Make sure all your connections are good. If they are try a different amp.