How to isolate the car ground with MCU ground?
I think your effort is seriously misguided. From your other question it looks obvious that you want to interface a logical signal in the range from 10 to 28 V to a 5-V tolerant MCU input. Optoisolator is not the device you need for this task, you need a simple signal limiter, something like a 1-k resistor with a 4.7-V Zener diode to ground. That's it, unless you have some other issues related to noise or something.