That'd be where to start for sure, and a really easy way to check if a ground is a problem is to take a long ground wire off the battery and go around the car with it.
Turn on whatever doesn't work and touch your wire to it's ground, and if it starts working you know the ground is the issue.
Since there is at least one working signal we know at least some of the brake/signal harness works.
First thing I'd check for the brake lights in this case would be the brake light switch or it's adjustment. (The brake lights and signals use the same wires from the dash back, so brake light power isn't getting there, but we know the wires work on at least one side)
