Replace speakers, add 4ch amp and keep head unit?

dont put under passenger seat, put under drivers seat, Never use the floor heater, and I did the same upgrade. I have a Polk 660 4chl amp. Just make sure you rum RCA converters from the HU first back to the amp, and the spaker wire back to the HU and match them to front and rear left and right from that point. I put polk 6.5 in all four doors and never touched the dash speaker.

 

Looks like a good amp. I loved all my old Alpine head units. Crutchfield definitely rocks.
The crossover is only a high-pass filter capacitor mounted on the dash speakers. You can directly replace all 4 door speakers with no issues as the dash ones are wired parallel to the front doors. Here's the link to the thread where I found the schematic and info... (Can't seem to be able to just upload it)

'12 Prius III speaker upgrades | PriusChat

You'll see all of the connections, wire colors, and even the designated connector pinouts at the end. I used an Audio Control LC6i to convert the high level signal into RCA's and mounted it under the passenger seat. It just fits but it's only about 6" x 8". You may have a tough time mounting your amp there with its dimensions since the heat vent may get in the way but at least there's a good bit of height to the seat bottom. I do agree that mounting the amp under the seat may be bad if it runs hot above and beyond how you set the car's heater. You may want to find a way to test it there, crank it awhile and see if it heats a lot. My LOC stays cool so I have no issues with the heater.

If you really want to save space, this little amp may fit the bill for you, it also has high-level inputs. I used Lanzar equipment back in the 90's and it seemed fine.... just an idea.....
Lanzar MNX460 (MNX460) 4-Channel Mini Max Series Car Amplifier

I tapped into the speaker harnesses at the door-body connectors. The front door speakers are in the connectors behind the kick panels and the rear door connectors are between the doors. I cut the wires and ran the HU-output side to the LCi6 and then the speaker-side from the amp. I found that the only color reference is on the end of the wire, so if you cut them, pay attention to what you cut or you'll have a pair of white wire in one hand and the color marked ones in the other! Before you cut, you can verify by buzzing the connections to see the 3-4 ohms.

Here's a few pics of my speakers with the adapters and baffles from Crutchfield. The other pic is one of the front door connectors. You can see how the colors are marked. I used spade lugs in a way that I can remove the amp completely and just hook the speakers directly to the head unit when I get rid of the car. The speaker wire is from KnuKonceptz, love it, ZERO noise at all with a cheap Boss amp in the trunk.

 

There is more room under the drivers seat as it is all flat flooring. I went with Polk 6.5 two way speakers. I made special mounts for the fronts and mounted the Polks on the old speaker frames. in the rear.

 

I had Polk DXi's in my last car and loved them but chose the Rockfords because they had a low power necessity, was originally not going to even use an amp. Had I known they'd be half as sensitive to the stock HU and I'd need an amp anyway, I would've bought the Polks! Can't go wrong when Crutchfield does the buy one, get second half off deal.

Crazy that all my other Toyotas had vents under both seats, I made an assumption and chose to throw the gear under the passenger. I see all the space now and I wonder why there's only one vent to the rear in this car. It'd be a great place for the amp but only if it'll stay cool though. Where is the oem JBL amp?

I like the idea of reusing the old speaker frame. It has to be easier than drilling new holes in the doors for new screws and clips. What did you use for the adaptor plate or did it all go right together?

 

The frames I made I used the old mounting holes and used screws and nuts, then mount the speakers to the mount. Just need to understand the + and - on the speakers and match them to the correct color wiring for the speaker . Easy, but now I'm thinking of a sub woofer for the back, never ends.

 

That's cool that it worked so easy.

It's true that it doesn't ever end!! Like modding, once we get the bug it doesn't seem to go away. My wife won't let me mess with her car but I've never had a stock stereo system last longer than the week it takes to buy up the new parts.