I just purchased some after-market seat heaters for my 2013 JK Sport S and was planning on running a new circuit from the fuse box to power them (rather than pulling a fair amount of current off an existing circuit). While not quite brave enough to run a new circuit the "right" way (pull the whole fuse box and push the new wire into an empty fuse slot from underneath), I was planning on putting a fuse in the M8 slot (front heated seats, heated steering wheel) and using a fuse tap. But when I popped open the cover on the fuse box, I found that M8 already had a 20A fuse in it.
Does the fuse being in there mean that I have a circuit off that fuse already wired into the Jeep? Is there any chance there are wires somewhere near the seats or center console already running off this fuse? If so, any suggestions on how I identify that circuit?

Thanks!

 

My 2008 has a fuse in that heated seat slot, too. There is no wire exiting the fuse box for heated seats, or anything else, though. The slot in the connector is empty. I don't know if your 2013 is the same way.
A fuse tap would work if you can find one of adequate amp rating.
You may be better off keeping the wiring outside of the JK's wiring, using a relay off the battery. The control side could be connected to any switched source so it all shuts off when the key is removed, in case you forget to shut it off yourself.

What I did was add the correct size wire to the fuse box plug so the circuit can be used for something in the future.

I used the same procedure for M8 as in this thread for M7: https://www.jk-forum.com/forums/jk-electrical-lighting-sound-systems-13/selectable-battery-accessory-power-source-189556/

 

Thanks. I figured it was too good to be true (having the wires I needed already in place). And I'm really a bit afraid of pulling the whole TIPM and trying to find the right connector to plug into the bottom to add a circuit as if from the factory.
But you did give me a good idea... My JK has an outlet (#2, running off fuse M7) in the center console. I don't use this for anything, so I could move the M7 fuse to make it a switched circuit and pull the needed current for the heaters off that circuit. Not the cleanest solution. But I'd need to have both heaters on "high" and be pulling 5+ amps through the outlet in the console to come close to maxing out the circuit.
I'll likely go this route, or the fuse tap on M8...

 

Well, so much for that master plan (moving M7 to make the outlet in the console switched). It seems that M7 really controls the 3rd power outlet (the one in the rear), which I don't have. The one in the console is protected by M36, and is not switchable.
I did find the installation instructions for the Mopar seat heaters. Interestingly they're designed to plug right into the back of the switched outlet in the dash.