what would happen if I had my cruise control set to 75 mph, but after being stopped at a light reactivated the cruise control while in first gear? would I have to shift gears for the car? would it even work?
On my 5-speed, the cruise control won't engage (or "resume") unless I'm already over about 40 MPH, and in either 4th or 5th gear. That's the factory installed system, designed to work safely with a stick.
If I engage it in 4th, and shift from 4th to 5th, it will disengage as I pass through neutral. I'd have to re-engage it.
A poorly-installed aftermarket system might not work so well.