Container, right front side of engine compartment, yellow oblong cap with a windshield washer drawing on it.
To refill your windshield washer fluid, you must locate the dispenser under the hood of your car. Near the oil, there should be another cap that has a picture of a windshield on it.
The washer fluid pump is on the bottom of the washer fluid reservoir.
The rear washer fluid come from the windshield washer fluid reservoir on your 1999 Ford Expedition ( according to the owners manual )
It is attached to the bottom of the washer fluid reservoir.
The windshield washer pump on a 1999 Ford Explorer moves washing fluid from the reservoir and sprays it onto the windshield. It is capable of spraying in a constant stream or in pulses.
On a 1999 Ford Expedition : The washer fluid for the rear liftgate is supplied by the same reservoir for the front windshield ( 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
There is no washer relay.
I am 90% sure that you have two reservoirs for fluid and two different pumps.
You must first remove the windshield washer fluid resivour, and the pump is under a plastic panel.
Just in case this question is missing the word wiper or washer fluid...See "Related Questions" below for that answer...
If Your Question Is That You Are Not Getting Much Water Onto The Windshield. Look For The Screen To Be Partially Stopped Up In The Washer Fluid Tank Or The Hose. Also You Could Have A Small Hole In The Line Not Allowing The Unit To Pull The Fluid To The Wipers.