Only 1 reservoir. Its shared between the front and rear washers.
There are 2 motors on the reservoir. Each one pumps fluid to its respective nozzles.
The refill bottle is in the front engine compartment.
Its quite big. You can access the lower portion of it- by removeing the screws for the wheel well and pulling back on the lines.
This is the reservoir on my 2010 Traverse- yours will be similar.
Note the 2 motors and hoses.
Or the pump went bad. If you have somebody actuate the rear pump, go under the wheel well and listen for the noise, with the car off. No noise is prolly a bad pump. Noise with no fluid is a clogged line. Of course, this is assuming that there is fluid in the reservoir.