I want to add these as well. My grandmother can't get into my wifes 4runner. These would be great.
I got a pair of AMP boards for my Tacoma and tried to see if they would work for the 4runner. The frame is different and the location of the brake lines makes it impractical. I spoke to some guys at AMP on the phone a couple of times. They made it clear that there were no plans to design these for the 4runner. These things are quite vehicle specific and the 4runner is a low volume truck. Doesn't make financial sense for them.
The wiring is easy though. The AMP ones don't have an on-off switch. They just work with the door. I added a switch to my set up, but even that wasn't too difficult.
I wish they'd make this an add on...