Finding 12v wires for vent fan

It's possible your unit has an option of a powered vent in that location. I would pull down the inner trim if any and see if you can see wiring there.

If no wiring you might have to get creative. Can you use a metal coat hanger to reach from the vent location over to one of the ceiling lights? If so they pull the light down and tap a wire into the light fixture and use the coat hanger to pull the wires over to the vent location.

Last ditch is to use some decorative raceway from somewhere like Home Depot and run the wire up a wall and across the ceiling.

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If you have a light fixture within a foot or so of the vent opening you can just use a coat hanger and bend a hook at one end. You're just going to be fishing through some Styrofoam to intercept a set of 12v wires. Then just tap into the hot side before the light and jumper it back to the fixture...same with the neg side.

Not a Keystone but mine had two loops of wire next to the vent, pos and neg. Just had to tap in to the loops. :)