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I did the same as you and installed Laramie seats in my Express. I ran a hot from the engine compartment to a fuse box I installed under the seat so it is completely separate from the factory wiring harness. Just tapped both seats into that. I was able to figure out which wires operate the heat and ventilation as well . I was planning on trying to install aftermarket switches but probably won't bother with it for a while.One note my seat harness didn't match the plug for new seats at all. The wires were different colors and I was worried about my airbags passenger sensor ect and didn't want any check engine lights so I actually took the harnesses out if my stock seats and installed them in the Laramie seats. I re upholstered them anyway so it realy wasn't hard at all. So each seat kinda has two harnesses.I left the stock harness in the Laramie seat as well to operate the power functions. It's a lot easier than it sounds.
My truck harness plugs were right for the big yellow plugs but they didn't have the power lead, just the ground. So i tapped off console power for that. I obviously didn't have the grey plugs for the heat and cooling but i was gonna do like you said and figure out which pin turns on the heat and cooling and make my own harness and add switches if i ever get that much energy. Just haven't made it a priority.
Why do you 4th gen guys keep posting new threads in the 3rd gen forum?