You don't. It uses the front reservior.
The rear window washer is fed from the front window washer tank in the engine compartment
On a 2001 Ford Taurus wagon : Open the rear liftgate , and the refill plug is under the passenger side tail light
it has a motor in the base and that connects to front and rear washer jets.
Open the rear liftgate on a Mercury Sable wagon and you will see the rear window washer fluid reservoir fill plug near the bottom of the passenger side tail light
It is the same reservoir as the front. There are two pumps.
On a 2006 Ford Expedition : The washer fluid for the rear liftgate is supplied by the same reservoir as the windshield , that is located in the engine compartment
The washer fluid for the rear liftgate is supplied by the same reservoir as your windshield ( located in the engine compartment ) on a 1999 Ford Explorer
Seriously? Ok, it takes from the same container as the front - there is just a line running to the back.
According to the 2010 Ford Explorer Owner Guide the washer fluid for the liftgate is supplied by the same reservoir as the windshield
iN THE REAR DOOR OPENING.
AnswerPage 150 in the 1999 Taurus Owners Manual says this: Checking and adding washer fluid for the liftgate (if equipped)The opening for the reservoir is located on the passenger side under the tail lamp.Refill this reservoir with the same fluid you use for your windshield.There's even a illustration.See "Related Questions" below for more
In the Mitsubishi Endeavor, the rear window washer fluid refill is typically located near the driver's side of the engine compartment. Look for a reservoir with a cap marked with a windshield/water symbol. Make sure to use the appropriate washer fluid for optimal performance. Always check your owner's manual for specific details related to your model year.