Are the harnesses installed per model year the same for all models/trim levels?

Seems like it would be for economy sake in manufacturing from the factory, but the reason for the question:

If you wanted to install an e-locker rear diff, is the wiring already there?

If you wanted to install factory fogs (in the bumper) on a base model (assuming you replace the switch assembly on the column, too), is the wiring already there?

No, they are not. There may be some common wiring between different trim levels, like you may find that some plugs for stuff that isn't there, but they don't wire the engine wiring harness for both 4cyl and 6cyl or the dash wiring harness for both auto and manual climate control or the body harness for both heated power seats and manual non heated seats.

They used to be this way in the 80s but not in the 3rd gens. You have to match the harness part numbers to get the same harness. They are different . So always match the engine harness to the chassis.

For example: I put a 1998 engine in a 1997 limited. I swapped the engine harness to match the 1997 chassis because they had a few slight differences.

The change from one harness all models was probably done to save costs in mfg. I was majorly bummed when I found this out as I liked to plug in the accessories that were missing from other vehicles that had then.

So the changed happened in the 80s at some point. At least on the main body harness. My guess is when the EFI went in as their wiring wasn't there in the carbonated models..

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If you have the factory black plug behind the kick panel under the steering then you have the abs+wiring in your truck all you would need is the LROR wiring harness to run it to the locker.
Most all 4runners have the wiring in place for the fogs. you'll have to swap out the non fog stalk for one with the fog switch on the stalk , install the relay and the fuse. For anytime fogs a dash mount switch would have to be spliced into a power source or ignition on circuit.

The 1996 to 1998 do not have any wiring for fogs. They were dealer add on. just like the rear hitches were an add on..

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(this is all for 99+) The harnesses are somewhat piecemeal. The front fender harness is the same for all of them - you can find the plug for the Limited outside temp sensor on an SR5, and they all have wiring for the fogs, etc. The wiring for the temps sensor isn't on the dash harness though, because the Limited has so many other differences for the auto-ac. My 2WD had the plug for the ADD relay. Most 4Runners will not have the locker-capable driver's side interior harness, but some did, etc. Basically each harness section has a couple options and they were put together in different permutations.

-Charlie

What about heated side mirrors?
What determines if the wiring is hot?

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There are proprietary harnesses for each model and year. It would be too easy if they had the same...