How do I switch out the wiring harness in a 1994 Chevy 1500 with a 1992 Chevy 1500 under the dash

same transmission? same engine? 98-99 was the cut-over year for changing small block chevy motors

The computer or powertrain control module (pcm) on the 94 s-10 blazer should be located under the passenger side glovebox you remove the lower glove box screws (under the dash) then remove the pcm retaining nuts to lower it out of the dash I have found this easier if you take the passenger side seat out to give you more room to manuver on some of these it may also be located on the passenger side fender under the hood & near the shock tower it is large with a few big wire harnesses goin to it each harness is color coded to fit where it goes into the pcm

Check the wire harness on the transfer case to make sure it's still plugged in. Check the selector switch in the dash. Check the hub sensor wire harnesses on the front wheels.
Check for error codes through OBD-II connector under the dash.
Good luck.

Your ground wire or adaptor/plug is not getting good contact. Try cleaning it.

this might help
1994 Toyota Pickup Car Security Wiring Diagram
Battery Constant 12v+ Positive Wire (+): White And White/Red
Battery Constant 12v+ Positive Wire Location: Ignition Switch Harness
Starter Positive Wire (+): Black
Starter Positive Wire Location: Ignition Switch Harness
Ignition Positive Wire (+): Black/Yellow
Ignition Positive Wire Location: Ignition Switch Harness
Second Ignition Positive Wire (+): Black/Yellow Or Black/Red
Second Ignition Positive Wire Location: Ignition Switch Harness
Accessory Positive Wire (+): Blue/Red
Accessory Positive Wire Location: Ignition Switch Harness
Parking Light Negative Wire (-): Red/Black
Parking Light Negative Wire Location: At Steering Column Harness
Parking Light Positive Wire (+): Green
Parking Light Positive Wire Location: In Drivers Kick Panel
Power Door Lock Negative Wire (-): Blue/White
Power Door Lock Negative Wire Location: At Door Control Relay. The door control relay, is located behind the driver dash speaker.
Power Door Unlock Negative Wire (-): Blue/Yellow
Power Door Unlock Negative Wire Location: At Door Control Relay. The door control relay, is located behind the driver dash speaker.
Door Trigger Negative Wire (-): Black/White Or Black
Door Trigger Negative Wire Location: At 2-Pin Plug, Top Of Fusebox
Domelight Supervision Wire: Use Door Trigger, Requires Relay
Domelight Supervision Wire Location: N/A
Horn Negative Wire (-): Green/Red
Horn Negative Wire Location: At Steering Column Harness
Tachometer Wire Negative Wire (-): Black
Tachometer Wire Negative Wire Location: At Diagnostic Connector. The diagnostic connector, is located on the passenger inner fender near the fuse/relay box.
Brake Light Positive Wire (+): Green/White
Brake Light Positive Wire Location: At Switch Above Brake Pedal
Factory Alarm Disarm Wire (-): N/A
Factory Alarm Disarm Wire Location: N/A
Factory Anti-Theft System: N/A
Factory Anti-Theft System Location: N/A

chevy uses a link from the key lock in column the the switch under dash, 1 or two possibilities providing you have not had the key lock out, 1 is the switch under dash has loosened and moved
2 switch under dash has failed

It's behind the dash panel just below the steering column. There are two relays for the fuel pump back there. Chande them both out.

where do i plug my wiring harness up at/

Do you need the engine harness, dash harness.? where is your problem?

You have to unplug the old radio. The radio isn't the part in the dash. That is just the control to the radio. The radio is actually under the dash above the accelerator pedal. It should have the power wires, speaker wires and antenna wire going to it.