I cannot find where to fill the rear windshield washer.

From the 2013 operators manual;

Push the windshield wiper lever forward to spray washer fluid on the rear window. The lever returns to its starting position when released.

The windshield washer reservoir is used for the windshield and the rear window. Check the fluid level in the reservoir if either washer is not working.

Replacing parts just to replace them wont fix a bad wire or part. Better to use a voltmeter and ohn meter to check if your getting power to the wires. You may have a bad connection or bad switch on the column for the pumps. Without checking the circuits it will behard to tell what it is thats wrong. Find a wiring diagram and trace the wires and components.

There is a electric motor that pumps washer fluid . Located in the bottom of the washer fluid tank .
Wash Operation (Front and Rear Wipers)
The front and rear washer systems use the same windshield washer fluid pump. When either the front or rear wiper is in the washer position voltage will be supplied to one side of the windshield washer fluid pump (depending on whether it is front or rear), while the other side of the pump is grounded.
At this point, the windshield washer fluid pump sprays solvent onto the windshield. When the front or rear washer is activated, the BCM receives an input from the switch that is activated. The BCM will control the wipers while in the WASH mode. The BCM will control either the front wiper relay or the rear wiper relay to turn on the wipers. The windshield wipers operate at low speed for 3 swipes and returns to the park position.

When turning on the washers do you hear the motor run ? Have some one try it while you listen at the washer fluid tank ! These pump motors do go bad .
turn OFF the ignition.
Disconnect the harness electrical connector of the windshield washer fluid pump.
Connect a test lamp from the windshield washer pump control circuit to a good ground .
Turn ON the ignition, with the engine OFF.
Press the front washer switch.
Does the test lamp illuminate?
YES or NO !

Sounds like you may need to change the washer fluid pump. They do go bad. It's located inside the washer fluid reservoir, which is in the front of the passenger's side wheel well, behind the front bumper.

they are two totally different things.the wiper fluid only cleans the windshield,the radiator fluid/coolant is for keeping the engine cool,
please do not mix them up as considerable damage could occur.
the coolant bottle has a bigger (normally) metal cap about 2.1/2 across.with a seal around the inside edge,whereas the washer fluid cap just pops off to fill it.please check your handbook.

Yes there are two pumps located in the wiper fluid reservoir. If your in a bind, you can switch the plugs and small hose that go to them because they are both the same. The upper pump operates the back, and the lower pump operates the front. The back won't work until it gets replaced, but it beats driving backwards.

what is P13K?

check the fuse first. dealer can hook up NGS Tester and get fault codes to see what is wrong.

Principles of Operation
NOTE: The generic electronic module (GEM) will not activate the rear wiper system if either the liftgate or the liftgate glass is ajar. If either the liftgate or the liftgate glass becomes ajar while the rear wiper is in operation, the GEM will park the rear wiper motor.
There are two distinct modes of wiper operation:

• Off Mode: In this mode, there is no rear wiper activity, and the wiper motor will be in the PARK position.
• Interval Mode: This mode is selected when the rear wiper switch is in either the Interval 1 or 2 position. When this mode is selected, the rear wiper motor is driven up onto the glass by activating the rear wiper UP relay. When the wiper motor HIGH/LOW/PARK position input reaches the high position, the UP relay is deactivated and the DOWN relay is activated to reverse the wiper motor direction. When the wiper motor reaches the LOW position, as determined by the HIGH/LOW/PARK position input, the GEM deactivates the DOWN relay and will begin a new wiper cycle 1 to 10 seconds later, depending on the interval position selected.

To engage the rear window washer, rotate the rear window washer selector downward to the WASH position. The rear window wiper will run as long as the WASH position is selected. When the rear window washer selector returns to the OFF, INT 1 or INT 2 position, the washer will stop immediately, but the rear window wiper will continue for two to three sweeps and then return to its previous mode of operation.
The low washer fluid indicator chime warns that the washer fluid level is low. The low washer fluid indicator chime is a single tone with a duration of one second. The chime will sound when the low washer fluid indicator is initially activated.
The low washer fluid indicator warns that the fluid level in the washer reservoir is low. The low washer fluid indicator will illuminate when the washer fluid level is low and the ignition switch is in the RUN position.
The low washer fluid indicator will deactivate when the washer fluid reservoir is filled and the ignition switch has been cycled from RUN to OFF, then to RUN.
Inspection and Verification

1. The wiper/washer system is a generic electronic module (GEM) controlled system; refer to Section 419-10 .

1. Verify the customer concern by operating the wiper/washer system.

1. Visually inspect for obvious signs of mechanical and electrical damage.

Visual Inspection Chart Mechanical Electrical

• Hoses to windshield washer pump
• Wiper linkage

• Fuses 6 (5A), 8 (5A), 15 (5A), and 20 (5A)
• Wiring harness
• Loose or corroded connector(s)
• Relays
• Motors
• Multifunction switch
• Washer fluid level switch

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• The wipers are inoperative — rear

• Fuses 6 (5A), 8 (5A), 15 (5A), and 20 (5A).
• Circuitry.
• GEM.
• Multifunction switch (wiper/washer switch).
• Rear wiper up relay.
• Rear wiper down relay.
• Rear wiper motor.

This comes directly from the owners manual.
Checking and adding washer fluid for the liftgate
Washer fluid for the liftgate is supplied by the same reservoir as the front windshield washer.

U may have a kinked supply line at the tailgate.

I'm pretty sure it's the same as the front washer, You can check by see ing if there are two pumps in the front washer resevoir, if not there is a possiblity that there is a bottle in the back, servicable by opening the rear door and looking at the walls behind the tires. I hope this helps

Inside the Reservoir Under the Hood there is a Small Pump that needs to be Replaced.