To replace a Blazer wiring harness, first disconnect and remove the battery, setting it a good distance from the vehicle. To remove the original harness, disconnect the main connection from the firewall connecting to the vehicle's main computer. Remove the rest of the harness connectors and sensors using a small screwdriver and socket for the clips, retainers and bolts on the alternator and starter wiring. Next, jack up the car, and remove the remaining harness connections below the engine.
Before starting the replacement, take several photographs of the wire connections and placement. Use labels or masking tape to tag each wire according to its color code. Once the old harness is removed, label all the new wire connections according to the photographs and labels from the old harness before installation. Untangle any additional wires and study the manual and photographs to have an idea where each belongs. Carefully install the new wires.
Connect all plugs before placing them in the proper conduit, or on the chassis or engine. This also allows you to avoid incorrect wire lengths or split points for obstacles or connections. Once each connection is in the proper conduit, lower the vehicle, and reconnect the battery. Test the connection by starting the engine and inspecting the wiring. Look carefully for any wires that are too close to moving or hot engine parts, making any needed adjustments once the engine is off.