Hi gem, are you definitely putting the cable through one of the grommits when the window sill has been removed? I say this as I initially tried putting it through another hole which you can see when removing the black foam piece by the bonnet hinge. This seemed to keep the cable in the wing somewhere as you have mentioned, so just wondering if you've got the right hole?
Once the window sill plastic has been removed, there are two grommits as far as I can see; a big circular one and a smaller one that is kind of oval shaped with two protuding rubber bits on it.
The second one looks ready made for a hole to be drilled into for a cable to fit through. I used this grommit instead of the large circular one as it looks a bit neater.
Also, just as another thought, are you removing/lossening the glove box so that you can see behind it (should be able to see loads of wires etc) ?
I will post some pics to help as it deffo isnt worth paying £100 for. Once you find the right hole it'll be a 2 second job to get it inside the cabin.
I'm planning on finishing off my ICE install on Wednesday this week, so will take some pics of the holes for you.