I finally put a stereo in the 4Runner... Mostly to fill the hideous holes the previous owner hacked into the sides of the cargo area for 6x9s. I decided I didn't need a traditional stereo though. I prefer the row of switches and cubby hole I have now.
I used a Bluetooth module directly to a 400W amp in place of a traditional stereo. My phone acts as the stereo. I can stream Sirius XM, my favorite local FM station (through their app), or even Netflix through the speakers. Then when I'm wheeling and don't have a data connection, I have my music collection saved on my phone... No data needed.
I used marine rated Polk Audio speakers The front speakers are 4" in the factory holes. The previous owner had his 6x9s mounted right to the flimsy plastic panels in the cargo area. That wasn't going to hold up to the high power 6x9" Polks I was putting in.
I cut out some aluminum panels and riveted them in place. This way I could reuse the holes from the previous owner, but have my speakers solidly mounted.
Thankfully I used a nice Kicker amp with some extra features that allowed me to tune the sound nicely. If you try a similar system, don't cheap out on the amp. Since you're playing straight off the phone, you don't have all the sound controls that you get with a typical stereo.