I've got the old V6 pulled out. It will be a bit before I get a 5.3/6.0 (haven't decided yet) put back in. In the mean time I'd like to get my wiring figured out. What has to stay and what can go? I don't wanna pop the hood and see a mess of wires everywhere. I'll be going Fuel Injected also...

 

Remove the Engine Harness....Remove glove box liner, Disconnect PCM & all the other Connectors that connect the Engine Harness, Remove the 2 screws on that hold the Pass Through to the firewall, Drag all that out of the truck.
On the Drivers side, Disconnect the Bulk Head Connector, And IIRC a couple connectors need disconnecting to get the Engine Harness out.
The main goal here is NOT to cut anything!

Put the entire harness in a clean trash bag, Put the harness & bag in a Freezer over night....No I'm not insane!

The reason for freezing the harness is to remove the Glued together Pass Through.... Remove the harness from the Freezer, Take a hammer (Taking care not to damage any wires & Wear safety glasses!) Smack the Pass Through repeatedly 'til the plastic body & glue can be removed from the wires. This has to be done quickly while the glue is still rock hard. Now you can Cull out some wire you need without cutting them!

Cull out the compressor controls, Blower Relay hot lead, Fuel Pump relay trigger wire...There are probably more you need that goes through the Pass Through but cant think of them now. The Pass Through ID number is P101 if you want to do some research.


Drivers side Bulk Head Connector...
Cull the Oil Pressure sender wire (TAN) & Water Temp wire (Green) out of the engine harness. I hope you saved the Senders off your old engine?
A Couple Red (10 Gauge) primary wires that power the inside of the truck also run through the engine harness, Coil Ignition Hot (Pink). Fuel Pump power (Grey), Alternator Field wire (Brown).
The easiest thing to do is...Cull ALL the wires from the Driver side Bulk Head/Connectors from the engine harness.

Doing this will eliminate the old TBI engine management stuff you don't need & leave the stuff you do need without cutting wires, Of course cutting them is easier NOW.....But all the Splicing later will be very time consuming.

 

I just did this same swap. There are a few wires you need to keep out of the drivers side harness, so don't just cut them. The wiper motor, tail light, fuel pump, and sending unit wires are in the drivers side harness.