Not positive about your model but this should be able to be done through programming your ECU (maybe not the correct box name).
These settings should be analogue and not operating off a switch to work along side your overall engine control system.
Hopefully in 1990 models it indeed is this way for you. Makes it easy to do
Pretty sure I’ve read threads here where guys have done this reprogramming. Just not sure about C4s.
Let us know what you find out.