* Consult your vehicle's repair manual: This is the most reliable source. The manual will have diagrams showing the exact location.
* Look for a small, usually black, sensor with a vacuum hose connected to it: It's often near the throttle body on the intake manifold.
* Search online for images: Search for "1990 Chevrolet Cavalier MAP sensor location [engine size]" on Google Images. This might give you some visual clues.
Without knowing the specific engine size of your Cavalier, a precise location can't be given.