That vacuum source is usually a fitting on the intake manifold. It's a small port, often on the top or side of the manifold, near the carburetor (if equipped) or throttle body (if fuel-injected). The exact location varies slightly depending on the specific engine and intake manifold design. You'll need to visually inspect your engine bay to find the correct fitting. It's often a small nipple or barb.
Important Note: If you're working on your vacuum system, be sure to check the condition of all the vacuum hoses. Cracked, brittle, or disconnected hoses can significantly affect engine performance and fuel economy.
