No Brake or Turn Signal lights 69 Barracuda

It does sound exactly like a bad ground.
One thing I like to do in cases like this is to run 15 foot long ground wire directly off the battery, and go around touching it to the devices that dont work. (like bulb sockets and such)
It gets obvious fast if there are grounding problems.

The only other thing might be the connector under the left side kick panel may not be connected.
Also just so you know, most flasher units will not flash unless there is sufficient load on them.
So the fact that the fronts come on but don't flash tells us that the rear bulbs are not connected to the rest of the system. (loss of pos or ground either one will cause this)