Upgrade 1-CD stereo to 6-CD stereo with cassette

Only after you bring the harness from the old car (JBL equipped) and transfer it into the new car (non-JBL equipped). OR clip the ends of the harness that plugs into the JBL head unit and amp, and run new wires to and from the harness to the speakers, amp, and headunit.

Better to just do one of these:
1) Stealth install an aftermarket stereo in Prius | PriusChat
2) How To: Upgrade Head Unit (Kenwood eXcelon DNX6960) | PriusChat

 

Not knowing how the JBL audio harness runs to/from: headunit/amp/speakers, it might be difficult to remove the factory harness completely. This might not be the case, as you are confident you removed the JBL harness in its entirety.

Had the JBL harness been to difficult to remove completely, it would probably be easier to clip the ends of the harness, where the connector plugs into the head unit and JBL amp; obviously leaving enough wire to reattach your wire. Then you would be left reconnecting the cut ends and running the wire where it needs to go, which would be a different PITA.

 

I have not modified my factory non-JBL radio. The following info below, is based on my days of spending too much time/money tinkering with car audio in the past.

Non-JBL HeadUnit.
Likely one connector going into the unit. Wires going/coming from connector will be 12V+, 12V switched, Illumination, GRND, 8 pair of wires (Front Right, Front Left, Rear Right, Rear Left). Front door tweeters make their connection at the Front Door speaker, and have a capacitor adjacent to the tweeter, for the frequency cut off.

JBL HeadUnit & Matching Amp.
Likely one 1 square/rectangle white connector going into the unit. Wires going/coming from connector will be 12V+, 12V switched, Illumination, and GRND. You will have to solder these to the appropriate non-JBL harness.

There will likely be a cable that goes from HeadUnit to JBL amp. This is will likely be
12V+, 12V switched (AKA Remote Turn On), GROUND, and audio signal (for amp to amplify).

From the Amp, there will be likely be 9 pair of wires, if sub is a separate component. FR, FL, RR, RL, Sub. I forget if the sub is integrated into the amp or a separate component. If Sub is integrated into amp, 8 pair of wires will come from sub to go to the 4 door speakers.

The 8 pair of wires from amp (the door speakers) would need to be soldered to the original non-JBL radio harness, to power the door speakers.

All the JBL door speakers must be brought over to the non-JBL car.

You may begin to appreciate why it may be difficult to make a non-JBL car a JBL car. Nothing is impossible, but can be challenging.

If it were me, I would do the stealth install.
Cheapest route to get good sound.
Plenty of DIN size head units that are feature packed: Bluetooth (stream and cell phone talk), USB, Apple/Google capable, steering wheel capable, etc. Superior to factory JBL. Feature packed models will cost a little bit more.