Traction battery air intake vent....

Its under the passenger side seat at the back. The opening points forward.
I thought the manual's picture was pretty clear?

 

is it on the floor beneath the seat? interesting location, considering someone could easily stuff something under there without the owners knowledge.

 

The picture isn't clear to me as from it the possibilities that come to mind are it could be on the bulkhead behind the rear seat or in an extension under the seat itself on on the floorboard.

What I think the OP is really asking is how do you gain access to it and how do you make sure it is clear.

I just went and looked and I took off the right rear headrest, folded the seat back down, slid the seat forward and felt under the seat from the rear and there appeared to be a tunnel projecting forward from the bulkhead to about 8 inches so it is under the seat and the tunnel is just above the floorboard. It felt like there was a vertical plastic grill on it with rather large holes. Just make sure some rag or trash didn't fall down there and block it.

 

Here are pics of the battery vent grill under the passenger's rear seat. The first is looking under the front of the seat; the second is with the rear seat moved forward looking down from the back. It is easy to reach the grill from either direction, although the seat itself could be removed quickly with a 10mm socket used on four bolts.