Microcontroller output pin to pull 12 volt signal low
That will not work.

You say you have a pull up that you need to pull low.

In the way your schematic is drawn, whatever you do to turn Q8 on, will make it pull your signal high, as in, connecting it to 12V.

What you need is actually a simpler open collector (or open drain if using a fet) output:







simulate this circuit &ndash; Schematic created using CircuitLab

When MCU pin is high, it turns Q1 on, making it conduct and connecting the signal to ground. When you turn it off (MCU low), Q1 stops conducting, and the pull up pulls the signal back to 12V.