Just fitted amp to one yesterday and yes the is absolutely murder to get the cable through.
I put the through that grommet, you just need to find a way through and once you have feed as much cable through as poss, then take the water bottle off, only 2 10mm bolts and put your hand inside the heat shield and the cable should be there. IT is very hard to get through the bulk head tho, i would suggest that you drill a hole through the grommet where the bonnet release cable is just make sure you dont hit the cable or remove it first as Bambam said. If you drill through the grommet then when you pull the cable out should, you wish to uninstall the stuff you will not have a hole as the grommet should more or less seal back up.