The windshield wiper relay switch, on your 1999 Cadillac Deville, are located mounted to the firewall in the engine compartment. The relay switch will be next to the windshield wiper motor.
The fuse is next to the parking brake. If it has a relay, its in the under hood relay box near the windshield wipers.
check the fuses and relays
Ignition Key switch is bad.
because it doesnt work
Wiper park switch is bad
On the back of the engine right below the windshield wipers
Windshield wipers don't work
Why are there windshield wipers inside the car?
I got the same problem did you ever find the problem
either under hood or in trunk on back of seat. may be a relay
Replace the windshield wipers.
under the hood near the wipers is the relay box a location I experienced (second author here) is one or some of the three relays a few inches from the battery connections and I heard there may be one near the glove box (I tapped on a stuck one of the three to get the cooling fan to go off)