Sounds like a fun project. I can imagine a big stereo would put more strain on the 12V system, I've heard of some cases where people will install a secondary 12V battery for car audio, so that might be something to consider.

I've thought about putting in a better unit in the car myself, and fitting a backup cam etc. just give the car a more modern touch, but right now I'm happy just using my iphone connected to the aux jack!

 

So I’ve since upgraded and decided to really check out the specs for the wattage capability of the hybrid charging system and post what I’ve found here and I’ve calculated that the nominal 244.4 volts at 6.5ah comes to 1,589 watts RMS and a max voltage of 650v which comes out to 4,225 watts again that’s MAX voltage with some sort of voltage boost technology in the Toyota hybrid system. I’ve upgraded the engine and 2x 12v battery grounds to 1/0 100% copper shielded wire and plan on upgrading the dc inverter to 12v + cable to 1ga as well running on a Skar SK2500v2 monoblock amplifier running at a .5 ohm load on a Skar ZVX 12 v2 subwoofer pushing 3,250 watts at 14.1v and a Kicker 250 watt on 4 ohm for door speakers for a total of 3,500 watts for the sound system not including the electrical systems running the car. It may be worth it to upgrade the grounds on the hybrid ev pack as well..... nothing’s blown up in the year that I’ve had it like this nor any “check hybrid system”, noticed my headlights dim ever so slightly when at a stop with the radio at full tilt might just be because the car is struggling to stay together from all the bass.