It could be low water pressure or a partial blockage in an inlet valve.
Binding in the linkage, bad windshield washer motor
beacase it does.
Same place as you fill the front washer fluid. There is only one reservoir.
It is pumped in from the front washer reservoir.
where to fill washer fluid back window 2012 kia sedona
Most likely cause is the incoming filter screens. Where your two hoses fit on the machine the inlets on the washer have small fine stainless steel filters. You can pull these out with fine tweezers or thin pointed knife, and clean them. ( New ones are cheap if you crunch them) I clean them with an old toothbrush.
My 98 impreza just pumps from the front washer tank to the back
take it apart and replace gasket
You fill the reservoir under the hood.
Fill washer resevoir - if it does not shut off possiby a bad sensor
Fill the washer fluid or repair the level sensor in the washer fluid bottle.
it is at the top left when you open the trunk