if you've checked all the above it might be that you've blown the rca output on your HU, surprisingly common.
Easy to check this out, just plug a headphone jack to rca lead into the amp and a portable cd/md/mp3 player, switch it all on and see if it works. If your HU is under warranty it'll be a free repair although you'll probably have to live without it for 2 weeks, if its out of warranty it'll prob cost £50-100 to get it sent away and fixed.