butt cooler...
So IF, you really want to go to the trouble, both my wife's (now gone) Mercedes and my brother's BMW used a small blower fan that pulls cool air off the floor under the seats and pushes it up into the foam of the seat, and out.
The retrofit issue is that the 4Runner seats are not permeable foam. The BMW and Merc. seats have foam that allows the air to be pushed into and through the foam and out through the perforated seat covers.
small fan, no duct-work involved, simple on off switch.
I can't imagine that the end result would be worth the cost and frustration...
Good luck if you decide to go for it...
AMLOR