Read voltage from standalone car ecu help advice please(noob)

Hi all,
I have trawled the internet and can’t find an answer to my set up,
I have a standalone car ECU that enables map switching via a rotary switch,
I would like to display the different map settings on a screen,the switch is a resistor stack that enables up to 12 positions(currently set to 8),it conects to the ECU via 2 wires,one to an input with a 5v pullup and the other to ground,it is configured in the ECU software via the reference voltages in the different positions 0.2v 0.8v 1.5v etc,
I have replicated this by connecting the switch to an input with a pullup on arduino and reading the different inputs for the different switch positions and printing to lcd,
My questions are,what would be the correct(if any) way to wire from the arduino to the switch to read the voltage when it is connected to the ECU,this is more of a wiring electronics question,not the coding,
Can i connect without altering the reference voltage the ecu sees too much,if it is only slight i can compensate in the ecu software,how should i connect safely as to protect the ECU as it was horrendously expensive,and therefore i don’t want to ‘just try’ hence seeking advice from people that know better than I,simple to some of you no doubt but not to me,just want to be right first time,any help appreciated,thanks