Rear speakers or headunit?

Those speakers were like that from the factory. Common issue.

To do it right you need an amp for the back with a speaker level input. I believe there is a crossover built into the head unit that limits the rear speakers to low frequencies which are rarely output by the head unit except occasionally on hd radio.

With that said, some have swapped to a 2015-17 headunit but that requires some adapters. @gromittoo is the expert.

 

You can always improve on the speakers, but fundamentally the issue is the head unit.

The rear speakers are positioned down low and are somewhat muffled by the seats.

The front dash and door speakers per side are paralleled. The rear speakers have their own outputs.

You will lose Prius specific screens and functions, including some setups and hidden diagnostic screens. None of which are deal breakers. Not sure what happens when the head unit's network connection to the vehicle is lost. It may throw a code.

If the existing head unit is a 57011 it will have an expansion box attached to the bottom which provides GPS, bluetooth and navigation.

There are up to four antenna inputs and one proprietary rear camera input.

If I recall correctly, some of the antenna inputs require adapters. I don't think you can retain the factoty rear camera function with an aftermarket radio.

If it were me, I would consider the Toyota upgrade coming from a 2015 through 2017 v.

Again, I would contact @gromittoo for his first-hand experience. I don't believe he monitors this forum on a regular basis.

Finally, the low cost and full function upgrade might be an amp for the rear speakers.

Rear view 57011

 

The factory speakers are a total joke. Think old school clock radio cheap.

Upgrading the speakers will improve the volume, even though the wires to the speakers are like 22 gauge. Will need an adapter kit to mount aftermarket speakers, as the factory speakers are riveted in place.