Momentary input to constant 12V output?

Is there a cheap device, or a way to build such device, that uses a momentary input to provide a constant output? It would be used in a car's 12V circuit. That momentary switch usually sends a signal to a control module, but as the car is not originally equipped with this option, the module is not designed to receive the ground input. I'm looking for an alternative other than replacing the ≈$700-1000 module...

I'm confused by your statement that "the module is not designed to receive the ground input". What does that mean?

I am also not sure quite what you want or how to accomplish it, but I might mention that there are things called "latching relays". A quick flick of current and they stick in the "on" position until you send a re-set signal. I think it can be done with transistors to. think, 'monostable multivibrator" or "flip-flop" chip. If you explain better, the smart people can probably show you how to do this.

The Module in the car is the canadian module, made for canadian features. The europeans versions have rear fog lamps, wich are controlled by a 3 positions fog light switch - off, on (activates the front fogs) and the momentary position wich turn on the rear fog and return to the on position when released. the rear fogs turn off when the switch is turned to the off position or the key is removed. the switch is to be installed to control aux lamps. I usually can do anything with car electronics, but this one is just too much.

Why don't you wire up a relay to be self latching when you push the button and connect the relay so that it turns off when the ingition is turned off.

Yeah, I've been thinking about that yesterday night, and worst: I already tried it in the past, by mistake... How stupid! Thanks for your help guys.