this Isnt for a simulator, its for my engine swap that I've done 9+ years ago and finally want reverse lights to work.
@Wawa thank you for the octocoupler idea.
I had several 4n35 from a previous project. I had an old USB cord for testing with 5v power supply power bank. When testing, it had about 170Ωwhen connecting ohmmeter ground to pin 5 of the chip. I wasn't sure that would be good enough so decided to try it since its only ground signal. Not sure if a different octocupler would have a lower ohm value + this is in a cheap cheap breadboard that might be contributing to it.
5v power supply + 370Ω to pin 1
ground to pin 2
instrument cluster (waiting for ground ) pin 5
ground to pin 4
when pin 4 is connected to ground, It worked when using a 5v cigarette lighter (so it shares chassis ground)
now to find a resistor value for 12v to power the octocoupler and test again
thanks for everyone's help and ideas
clint