5 1/4 also go in the rear pod units, you may have to cut back the plastic a little, then you have to get creative to fix the pod to the shelf, I used Kappa Infinity Perfect speakers, which have a hole in magnet at the rear of the speaker, which I used to screw the original bendy-spider bracket on, so basically a standard fit. But most speakers you buy won't have a convenient hole in the magnet, so you will have to find a way to hold the pod down.

OR you can install speakers directly into the hole in the shelf - which is going to be a bit of an odd size I'd have thought, and you have to then find a way to screw the speakers down - into a metal shelf with the rear windscreen above it!

So all in all not easy.