When I got my 4Runner, I was excited to have some more room inside. My old standard cab pickup had almost no room. However, I found that the 4Runner did not have a good place to mount my amp. It was too large to fit under a seat. I drove it for months with the amp sitting on the floor behind the front seats. That was unacceptable. It needed a permanent home
I looked at what others had done. Some people mounted their amps to the back of the rear seats. That didn't appeal to me because I wanted to keep the cargo space in my 4Runner and I wanted the amp out of sight. There is a lot of "wasted" space between the inner and outer body walls on the 4Runner. I decided to utilize some of this space by mounting my amp in between the body walls.
I mounted my amp to a thin piece of plywood, and bolted the board to the inner body wall using bolts and t-nuts. Two of the bolts were inside and can be seen circled in red. The other two bolt though the inner body wall underneath the 4Runner as indicated by the red arrows.
Drilling the lower holes was a bit tricky, as you have to do it from underneath the 4Runner. To get the holes aligned properly, I first drilled the two top holes and bolted the board in place. I also used a clamp to hold the board firmly against the inner wall. Then, I did my best to estimate where the lower holes should be, and I drilled through the inner body wall and board from underneath my 4Runner using a small drill bit (just in case I missed).
Here you can see the amp mounted in place. Once the panel is put back in place, you can't even tell that the amp is in there.
I was concerned about heat buildup in this confined space. The amp does get rather hot, but it's been in there over a year and I've never tripped the thermal shutoff. It is a 50Wx4ch amp. Obviously, you need to be careful what you put in there, as some amps put off a lot of heat.
You can pull out the plastic pocket in the inner panel and feel the amp to see how it is doing. In fact, you can leave that plastic piece out to allow some fresh air to get in there. I am thinking about cutting some vent holes in the plastic piece and perhaps mounting a quiet fan in there to get some more air circulation.