Where does the vacuum line enter passenger compartment that controls AC vents on an 87 Chevy Astro Van?

There isn't a single vacuum line directly controlling the AC vents in an '87 Chevy Astro van. The vacuum system controls many aspects of the HVAC system, including blend doors (hot/cold air mix) and possibly mode doors (dash vents, floor vents, defrost), but it's a complex network. The lines themselves don't enter the passenger compartment in one easily identifiable spot.

Instead, several vacuum lines run throughout the dash and firewall area, connecting to various actuators. To find where the relevant vacuum lines enter, you'll need to:

1. Consult a shop manual: A factory service manual for your specific year and model Astro van will show a diagram of the vacuum system. This is the best way to identify the components and lines.

2. Trace the lines: Start by locating the vacuum actuators behind the dash that control the blend and mode doors. These are usually easily identifiable small, canister-shaped devices. Follow the vacuum lines from these actuators to find where they pass through the firewall or other openings into the engine compartment.

3. Check common entry points: Look for vacuum lines entering the passenger compartment near the firewall, typically around the area where the HVAC system is located behind the glove box or under the dash. There might be a grommet or seal where multiple lines pass through.

Remember to be careful when tracing vacuum lines; they are often fragile and easily damaged. Disconnecting any line requires careful attention to avoid mixing them up during reassembly.