A couple of places to start.

In the manual, it looks like two common ground points for the ignition coils are to bolts in the intake manifold. Look at the intake manifold bolts towards the rear of the engine (standing in front of vehicle) between the fourth and fifth runner tubes.

One the passenger side, there should be a BLK/BLU wire with other wires connected to the same bolt. One the drivers side there should be a BLK/RED wire with other similar wires connected to that same bolt.

Another common ground in that circuit is at connector H-5. That connector is located on the drivers' side of the engine compartment at the rear of the brake fluid reservoir (looks to be about even with the middle of the reservoir).

Making sure the connections at those two points at tight/clean might be a good place to start anyway.