I have the same problem, I have an aux-in cable plugged into the back of my HU for a DVD player mounted right below and they both hit the white plastic housing behind them. I'm not 100% sure but I think that the housing back there is for the a/c ducts in the center of the dash. I don't think cutting it would be a good idea either, you can maybe sand or grind it a little thinner than it is tho, just don't go all the way through...