Are you sure there's no hole for the ground wire?
The kit should come pre-drilled with a hole to the control rout, positioned under where the bridge will go (highlighted in red). It's at an angle, so can look more like a mark than a hole.
If there isn't one, then you'll need to drill one, and you'll need a very long drill bit to do that. loom for a 300mm long bit. Partly because its a long hole and partly so the drill doesn't get in the way and you can get the angle correct I'd suggest a 4mm bit. It won't t be easy to get the angle right. You'll need to drill from the top to the rout, you won't be able to drill the other way. It may be cheapest to buy a set, something like this.
https://www.amazon.co.uk/300mm-Extra...NsaWNrPXRydWU=
Drill a small depth vertical hole first, otherwise you'll never get the drill bit to bite and not skid over the surface. I'd also suggest cutting out a wedge of the correct drilling angle from scrap wood that you can lay the drill on top of after securing it in place with double sided tape, and use pencil lines to get the direction right.