I somewhat solved it. The AUX input does support video, and the input shows up on the AV input menu. I can use the steering wheel controls to scroll to the AUX input, and see my front camera. I never did get the second camera input working.
If you do this, you need a 3.5 mm mini-jack to RCA adaptor. The minijack plugs into the head unit, next to the antenna input. Rather than pay $45 for the Pioneer minijack adaptor, I bought one for $5 on Amazon. Mini jacks are not all made to the same standard. I got unlucky, and it wasn't set up correctly, so I rewired it. The correct order f