Sub to Stock Unit? Not trying to beat a dead horse!

Hey guys I've been searching and reading all day but I still can't find some posts to answer a couple of questions I have. Help would be appreciated. I'm going to do the cubby fiberglass sub box. I already have everything but I'm trying to figure out if I should link to stock HU or go with the stealth install. A couple questions.

1. Can I hook up the aftermarket unit to the stock HU as an auxiliary so I lose no steering wheel controls? I know about the Crutchfield Factory Integration Adapter, but that looks like it's for more of a full switch!
2. Will a 4channel to RCA converter work with the Prius to tap into the speaker wires for the connections?
3. Do I need a tone converter to drop those signals down to low range sub levels? (not sure how that one works)
4. When playing music with the sub running from the 12V battery, will the car kick back on to charge that battery when it's getting low? I don't know if the 12V and the hybrid batts work in tandem.

I will do a full write up of my project once it's done to help other people answer similar questions in my situation. I'm not a newb to electronics, but this will be my first time trying to tap into speaker wires rather than using RCA hookups off an aftermarket HU.
If it comes down to it, I will just install the stealth HU and use those RCAs for the sub.
Thanks

 

Thanks Shovlhed, looks like I'll grab a new deck after all. I'll def check into replacing those front speakers too. The JBL seemed to sound okay to me but I've become contempt with that system. I have always been a bass head so to have a sub in the back would compliment my taste as well. I've been using the same P3 for years and it's traveled with me from car to car, it's about time I get it back in this car. I'll probably stick with my current amp, so with that being said do you recommend any converters? I've seen the Scoshe one, but it really looks cheaply made.

 

No longer interested in the converter. I've always wanted Sirius XM in my car so I'm getting a Kenwood X998. I do have the JBL system in my car so I'm compiling a list of things I need. A wire harness, a PAC Tato, a XM receiver cord and antenna, and an amp wire kit for the sub.
When buying an XM ready HU am I correct in thinking I need this

or do I just need the antenna? The wiring for these harness' and the PAC and the ability to control the radio from the steering wheel still looks like a nightmare..