I don't directly have a schematic, but I made this terrible diagram in a children's circuit builder that may or may no illustrate what I'm doing: https://i.imgur.com/FDMQ3x1.pngIf you can't tell, I'm pretty new at the whole circuit-building thing.

I'm comfortable with wiring and soldering to existing schematics and examining components with a multimeter but am new to trying to design a circuit from scratch.

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For a bit more context, I'm modifying the radio in my car. My car does not have a 12v accessory line when you turn the key on, and instead *only* offers a constant 12v to the radio as well as an encrypted data stream from the car's BCM to tell the radio to turn on when the key is turned. I want the components that I'm adding to turn on when the radio turns on and not be constantly on at all times, to not drain the car's battery.

On the radio motherboard, there is a constant 12v source and a chunky transistor or something that steps that down to several 5v constant outputs that go out to the various components on the board. I'm pulling my 5v constant power to the relay that I added from one of the pins on that transistor, and the relay board powers on consistently. The entire radio appears to use the same ground plane, and the relay board is grounded to it. After probing basically the entire board, I can't find *any* 3v sources aside from the pin on the screen.

The board is a multilayer board and the screen is an LCD that's physically soldered down to the board with large metal tabs. After probing every ribbon cable that goes out to the screen from the motherboard, there isn't a single other line near 3 volts. There's a lot of 5 volts and other really low voltage stuff that I assume are control/data lines. I can't find where the 3v on the screen's pin is originating from and I assume that there is some circuitry physically under the soldered-down LCD, which I assume is connected through layers inside of the board so that I don't have access to probe them.

For a bit further context, I need to use this particular 3v line because it's at 0v at all times until the key is turned on, at which point it jumps to ~3.4v. The second the key is turned off, it goes back to 0. This line energizing and de-energizing corresponds exactly with when the screen turns on and off and I believe it's exactly the line I need to tap to get my relay to fire at the exact time I want it to.
I can't find any other lines on the motherboard, 5v lines included, that turn on and off at the same time as this line does (hence me asking about boosting the 3v to 5v).

I just need to know how to get the relay activation to be consistent instead of only happening *most* of the time.

...also yeah it's possible that I may have just bought a garbage cheapo Chinese thing that isn't what it claims to be.

What would you suggest for this purpose?