There's a screw that holds on part of the metal trim on the right hand side of the engine bay (the right side if youre facing the car head-on). It should be right along the edge of where the bonnet closes. Just unscrew that, crimp a wire into an o-shaped clip or a u-shaped clip and then tighten the nut back down
you can sand down the metal beneath the bolt if you want to be extra sure that the wire will ground, but I've had a ground for my coolant temp gauge sensor there for ages and it works great!