Depending on how much you want to graft over, you will want the engine bay harness and the dash harness to start with. You may want the gauges as well as electric speedo's started in the late 80's.
I have found it easier to leave the wiring on the engine and pull the harness through the cab to pull with the engine. It will be easier to remove the fenders here, however not required.
The A/C and fan control are part of the engine harness. These are not needed to make the engine run - you should leave for now until you get everything from the donor. The engine harness will go through the passenger side firewall to the ECM under the dash. There is a horse-shoe looking clip and about a pound of silicone like stuff where the wiring harness goes through the firewall. The harness then connects to the ECM, dash harness and fuse box. Remove the clip, and chunk away the silicone and you will be able to pull the connectors from the inside through.
The dash harness has the ALDL and the check engine light, and may or may not be connected to the ignition switch (vehicle dependent). I would also grab the cruise control and even the steering column personally.
The fuel pump control wire and the fuel level gauge are run together - one is pink, one is brown (dont remember which is which right now) If you do not have a good 1Bar fuel pump, then I would also grab the fuel tank and tank wiring. At that point, the lighting wiring is the only thing left from the K3500 that may be of interest. Then again, painless wiring has universal kits.
I also recommend to get a good wiring schematic of the K3500 as this will be essential for trouble shooting.
Scott
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