Windshield wiper How do you time the windshield wipers on a 1996 GMC truck. I replaced the wiper motor and the wipers will not lay at the bottom of the windshield

Go to the link please. How To Install Replace Windshield Wiper Arm 1996 2013 Chevy Express GMC...

Sometimes you need to bend the arm to put more pressure on the glass.

a known problem with GM wiper motors were the circuit boards which may be replaced seperate on some models. also would cause what we call a park issue meaning not stopping at bottom of the windshield. hope this helps.

Windshield Wiper Motor
Removal & Installation
To Remove:

1. Remove wiper arms.
   
2. Remove weatherstrip from cowl.
   
3. Remove two cowl vent grille panel screws.
   
4. Remove windshield nozzles and hoses.
   
5. Remove three cowl vent grille panels.
   
6. Disconnect windshield wiper motor electrical connector.
   NOTE: Do not remove crank arm from windshield wiper motor.
   
   
7. Loosen nuts that attach the drive link brackets to the crank arm.
   
8. Remove wiper motor crank arm brackets.
   
   
   
   
   
   
   
   
   
   
   
9. Remove three bolts and windshield wiper motor from vehicle.

To Install:

1. Install windshield wiper motor and three bolts.
   
   
   
   • Torque to: 62 in lbs. (7 Nm)
   
   
2. With right side linkage closest to the windshield wiper motor, install drive
   link brackets to wiper motor crank arm.
   
   
   
   • Torque nuts to: 44 in lbs. (5 Nm)
   
   
3. Connect electrical connector.
   
4. Install nozzles and windshield washer hoses to cowl vent grille.
   
5. Install three cowl vent grille panels and screws to vehicle.
   
6. Install weatherstrip to cowl.
   
7. Install wiper arms.

On the wiper motor there is a function called the "park" mode. If the wipers are not returning to the bottom of the windshield the park mode of the wiper motor is malfunctioning. The most common fix for this problem is to replace the wiper motor.
If you have the ability to disassemble the wiper motor you should see a set of contacts inside the wiper motor transmission. These contacts get worn over a period of years and sometimes just a good cleaning will solve the problem.

It sounds like the wiper motor has a bad gear. Replace the motor and it should solve your problem.

Check for a loose fuse or loose wiring to the wiper. Also check for tight ground connection at the motor. The 'park' switch which completes each cycle may be intermittent. It might be a worn switch at the steering wheel.

you likely broke the plastic gear inside the wiper motor by turning wiper on when wipers were frozen to glass. you will have to replace the motor.
you can buy motor on Ebay cheap.
you have to remove screen at bottom of windshield for access. remove side windshield mouldings to be abe to get screen off.
watch very, very carefully how you get wiper linkage out or you will have a hard time getting it back in.

disconnect the wiper transmission from the motor, then run the motor and turn the motor off with the wiper switch, this will park the motor, re-attach the wiper trans linkage so the wipers are parked/down

This sounds like the park circuit is shorted.