You can find your 1999 Ford Expedition front windshield wiper fuse in the fuse box. The windshield wiper fuse will be in the second column, fourth from the top.
Could be he wiper motter or the fuse.
The 1998 Ford Expedition had a mid-year production change which affected the wiper system. See sources and related links below for the best explanation of how to determine correct wipers.
buy a new one
Go to your local parts store and they will help you.
On a 2000 Ford Expedition : The rear windshield wiper control is on the inner part of your turn signal lever towards the steering wheel column cover . It has OFF / INT 1 / INT 2 positions
where does the wiper fluid go in a expedition ======================================================== In a Ford Expedition : The windshield washer fluid reservoir for the windshield ( located in the engine compartment ) also supplies the rear liftgate
its pluged in to the wiper, light,indicator switch assembly.
Check Owner's Manual. If you do not have one. Check Related links below.
Take it to a Ford Dealer.
Remove the grill around the wipers, make sure to get all the screws, some are hard to find, but when you get all the screws the grill lifts off and exposes the wiper assembly. Remove wiper arms, nuts are under a cap at the base of the wiper arm. Unplug the electric assembly. Remove all mounting bolts from the wiper assembly and it will lift out. This works for a 2000 f150 so it is probably the same for 1998.
The 2004 Ford Expedition takes 20" wiper blades on both sides front and a 17" rear wiper. See sources and related links below for more information. Since there are over 22 different wiper arm attachments, do not buy wipers just by length only - be sure to lookup exact wiper blade part numbers by vehicle make, model and year for the brand wiper you decide to purchase. This ensures correct length AND correct attachment.