Replace the pump. Your local auto parts store will probably have one in stock.
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 2007 Ford Explorer : The windshield washer fluid for the liftgate on the rear is supplied by the same reservoir as the front windshield ( that's located in the engine compartment )
The washer fluid for the liftgate is supplied by the reservoir for the windshield according to the 2000 Ford Explorer Owner Guide
should be the same reservoir as the front windshield wipers
#12
Starting with the 1997 Ford Explorer the rear washer fluid is supplied by the windshield washer fluid reservoir in the engine compartment
On a 2001 Ford Explorer : The washer fluid for the rear liftgate window 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 the front windshield , located in the engine compartment
According to the 2010 Ford Explorer Owner Guide the washer fluid for the liftgate is supplied by the same reservoir as the windshield
I believe the windshield washer fluid container in the engine compartment serves front and rear on 2002 and newer 4 door Explorers
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
I have a 02 Explorer and the washer pump is on the windshield washer tank. So I would check there on your truck. Good luck.