Where is my wiper relay in my 1992 gmc step side bed

GMC what ?
Your best bet , take it to the GMC dealer . Computer controlled . The wiper switch is a low voltage input to the BCM - body control module . The BCM controls the wiper's by energizing two PCB relay's . These relay are non replaceable , soldered inside the under hood fuse box . BCM probably has DTC'S - diagnostic trouble codes stored .

WPR/WSW fuse 25amp is located in the under hood fuse box . This a complex system ,controlled by the BCM . If you find that the fuse is good i suggest you take it to a qualified repair shop . This wiper / washer system has the capability to set DTC'S - diagnostic trouble codes !
DTC B3715 02: Front Wiper Relay Drive Circuit Short to Ground
The Body Control Module (BCM) monitors the control circuit of the WIPER relay. The voltage level should be low while the WIPER relay is de-energized and near system voltage when the relay is energized. After the BCM receives a low, intermittent, or mist signal from the wiper/washer switch, it responds by applying battery voltage through the WIPER relay control circuit to the coil side of the relay, energizing the WIPER relay.
DTC B3873 00: Front Washer Relay Circuit
DTC B3875 00: Wiper High Speed Relay Circuit
DTC B3922 00: Front Wiper Function Select Circuit
I really don't think this is a fuse problem , but if it is a blown fuse they don't just blow for no reason . Low resistance high current flow ,short to ground .
The wiper / washer switch is a input to the BCM - body control module . Depending on switch position the BCM will energize a relay the WPR relay or the WPR HI relay .

Both headlamps an wipers are controlled by the BCM - body control module ! There is a lot involved in turning on the headlamps an wipers , here is GM factory service repair manual description .
Low Speed Operation
When the wiper switch is in the low speed position, ground is applied through a resistor internal to the switch and the wiper switch low signal circuit to the body control module (BCM). In response to this signal, the BCM energizes the WPR relay by applying battery voltage through the wiper relay control circuit to the coil side of the relay. This allows battery positive voltage from the WPR fuse to flow through the switch input side of the WPR relay and out to the switch input side of the WPR HI relay. Since the wiper high relay is de-energized and its switch contacts are normally closed to the low speed control circuit of the windshield wiper motor, the motor will operate at low speed.
Wiper motor low speed operation and the WPR relay can also be commanded ON/OFF by using a scan tool. Refer to Control Module References.
There could be DTC'S - diagnostic trouble codes stored in the BCM .
DTC B3715 DTC B3873 DTC B3875 DTC B3922
Your best option would be to have it checked out by a qualified repair shop . Hooking up a diagnostic scan tool to check inputs an outputs of the BCM is the best way to find the problem . Could be the switches (no input to BCM) The BCM could be at fault . There are relays involved in both the headlamps an wipers , couldn't be three relays bad at the same time .

I think there is a black box under the hood usualy on passenger side it should be in there.Look under the lid of the box for what relays do what.To test relay pull one from another spot and try it in the wiper spot.

check out light switch for return earth back to relays

Under the black panel under the hood on the passengers side firewall. there is a relay and a fuse there.

should be under the screwed down cover behind the fuses in the engine compartment.
If your wiper is having issues normally it is the mechanical side of the unit. Try going to www.benzworld.org or peachparts.com and look on how to lubricate it.

The fuel pump is in the tank, tank can be droped out to chg.pump, Its up to you how you want to do it but for me i always found it easier to unbolt the bed and lift it back about a foot and i could get right at it, and i did not have to mess with the tank straps and fuel lines, If that is the problem. Chk.fuses and relay first.

It sounds like it is the internal swiitch and not the relay. I would check the internal switch

Start by looking for bad or disconnected grounds. My best solution to the wipers is that when they dont want to work just raise the hood and tap the wiper motor with a screwdriver.