You really should disconnect the power fully- you are going to be the guy we find dead next to his Tesla. The connector under the seat can provide 200 amps at 12V. That's 2400W of power and it will kill you hard. Accidentally drop a wrench on there and you get free arc welding.
It's really easy to do this right:
1) Leave open the hood and doors
2) Power off car from service screen. Wait for clunk of high voltage connectors releasing. 5 minutes.
3) Disconnect the ground wire from the front 12V battery.
4) Disconnect the connector right next to penthouse bolt. It's a bail connector, open bail, pull up.
I've been studying this fairly hard for months now, and the reason the car throws codes is if it detects too much inrush current when powered up. So if your amp has giant capacitors for example, that initial power on can make the car think there is a short on the penthouse connector and it throws a breaker, and then the codes to report.
As long as you don't have heavy inrush at power up, the DC-DC converter doesn't really care about the draw. So for example if you can set your amp to power up only on input signal, that will be later than that first split second power up.
Any draw over 200A is going to throw the breaker as well, so that's the upper limit of what you'd ever want here. I'd recommend not going that high regardless, because the 12V system is used for other stuff in the car like the main computer and your driving lights. I personally am going to only set up a 1kw audio system, but if you want to push it it some, you could go as high as 180A or about 2000W. If you go that high it probably makes sense to set up with a stiffening capacitor and a relay to avoid the inrush problem. (Sorry missed earlier question- I don't know of best cap to use there.)
There is a thread from a guy using the penthouse connector to power a 12V-120VAC inverter, and running his house off the 2000W inverter. So we know that all works OK, even if Tesla will give you the side-eye. For stereo use, it won't be a constant 2000W draw, so it's quite a lot less than this inverter case.
For my setup, I'm going to use 2 Class D amps with max 90A draw that can powerup on signal, so I don't currently think I'll need to use the bypass relay. Of course if I ever get codes, I'll add it. BTW, if you ever get the codes, you need to reboot the car by pulling that DC-DC converter plug under the seat.
For what it's worth, I tried to read everything on the net about the penthouse vs. 12V front battery for powering the stereo, and my conclusion is that the penthouse is the best spot. The front 12V battery is more complicated than a normal car, because the Tesla is trying to detect early failures of that battery. When it goes bad it disables your entire car. Their monitoring system doesn't work well if you add extra draws to the 12V battery, and makes the car think it's losing power. Also, we know the 12V battery is a weak spot in their design, and I want to avoid putting any additional load on it.
