Delay wipers only work on high speed on 2000 chevy silverdo 1500 truck no delay

You didn't take into account the wiper switch is a input to the BCM - body control module ,or do you know that ? The BCM controls wiper one an wiper two relay's to turn on the wipers .
Wondering if it is the PCB or perhaps a physical problem? Guessing never finds the problem . Reading how they work in factory service repair info should be the first step for someone that doesn't know .
Intermittent Operation
Windshield wiper DELAY operation is a low speed wiper motor function with a variable delay interval between the wiper motor cycles. The DELAY interval is determined by a series of 5 resistors within the wiper/washer switch. During DELAY wiper operation, the signal circuit voltage is further reduced through the DELAY resistors. The body control module (BCM) monitors the voltage on the signal circuit to determine the delay interval between the low speed wiper motor cycles.
If the wiper switch is moved from a longer delay interval to a shorter delay interval, the BCM will command an immediate wipe cycle and reset the delay timer to the shorter delay interval.
Additionally, intermittent wiper operation is vehicle speed sensitive. As vehicle speed is increased or decreased, the wiper interval will also increase or decrease.

Low Speed Operation
When the wiper switch is in the Low position, voltage is supplied through an internal resistor to the signal circuit. The body control module (BCM) monitors the windshield wiper switch signal 1 circuit and energizes the wiper 1 relay which will operate the windshield wiper motor. The wiper motor will operate at low speed, since the wiper 2 relay is de-energized. Low speed can also be commanded with a scan tool by commanding Wiper Relay Cmd. ON and OFF.
High Speed Operation
In the High position, the windshield wiper/washer switch supplies voltage to both the wiper 1 and wiper 2 relay coils. This energizes both relays and allows battery voltage from the wiper 1 relay switched output to the switched input of the wiper 2 relay through its contacts and out the switched output of the wiper 2 relay to the high speed circuit of the windshield wiper motor. The windshield wiper motor will operate at high speed. During HIGH speed wiper motor operation, the wiper switch signal circuit is at the LOW speed signal voltage, and the body control module (BCM) supplies redundant voltage to the wiper relay via the wiper diode.

Wiper/Washer System Components
The Wiper/Washer System consists of the following components:
• Windshield wiper/washer switch
• Body control module (BCM)
• Wiper 1 relay
• Wiper 2 relay
• Wiper diode
• Windshield wiper motor
• Windshield washer pump
• Windshield washer nozzles
• Windshield washer solvent container
• Windshield wiper arm and blade assemblies
• Windshield wiper transmission
• WIPER/SW 10A fuse
• WIPER 25A fuse
Refer to Master Electrical Component List.

DTC B3715 02: Front Wiper Relay Drive Circuit Short to Ground
DTC B3715 05: Front Wiper Relay Drive Circuit Short to Battery or Open

Your best bet , take it to a qualified repair shop.

There is a wiper delay module under the dash that has failed. Windshield Wiper Switch MOTORCRAFT SW-5665

The first thing to do is drain and refill the transmission with fresh lub, it uses ATF (type Mercon 5) in all Ford manuals.

When they won't park it's 99% need new wiper motor

Replace the wiper module. Cardone at around $35-$50. This is the black cover under the hood, driver's side with 3 screws and about 4" x 4". Takes about 3 minutes to change.
A1 Cardone 81158PB Remanufactured Wiper Motor Pulse Board Module

try little circuit board on wiper motor

The wiper switch could be bad, but have it checked out by a shop, you dont want to guess and replace parts that are good.

wiper module which is on the wiper motor buy just the module

I had this problem (wipers only work on high) with my 2002 Chevy Malibu and had to replace the 6 amp diode (22059733) on the wiper control board. It is connected to the back of the wiper motor and is held on by 3 torx screws.
Radio Shack has them for $2.75 6A, 50V Rectifier Diodes (4-Pack) Model:276-1661

Chevy has had Problems with the Wiper Motors on these Trucks for a Long time now. On some Models they Issued Recalls. On My 95' I just Replaced the Wiper Motor and have had NO Problems Since!