Where is the vaccum canister on 1999 e-250 cargo van?

Hook the battery positive of the camera to the light green wire going to the reverse lights.

google images for your : 2002 ford Econo Van 2.0ltr motor fe vacuum hose diagrams.

under passenger door step

The vac tank is usually in the engine compartment, don't know about a check valve for the a/c unless you are talking about the HVAC control on the dash.

you have a bad vaccum line

check and see if you are getting vaccum to your vaccum ball under the hood.if you are then follow the vaccum line that goes to the inside of the vehicle until you find the vaccum leak.

This is a common problem on all Astro/Safari vans. The problem is a leak in the vaccum lines that control the vaccum switch responsible for changing air direction from the defrosters, dash vents, floor vents..etc.
Many of the problems that plaque the Astro/Sarfari vans and many other "Mini-Vans" is the build up of excess heat in the engine compartment. It\'s a tight place to put a V6-4.3 Vortec engine and the heat generated bounces back and cooks everything on the engine (coils, modules, sensors, wiring, and of course plastic vaccum hoses). No need to mess with your switches or anything behind the dash -- replace those thin plastic vaccum hoses to operate the vaccum switch (located just below the A/C canisters, to the right of the heater/ac blower housing) and your dash switches will magically send heated or cooled air where you want it.

the van has no choke its fuel injected, it will need scanned to diagnose the faulty sensor or vacuum leak

You have to drop the fuel tank. It is inside. Hope the tank is near empty, remove the 2 fuel fill hoses, you have two straps, lower tank, remove 3 wire connectors, and several more lines. Please use an ac delco fuel pump module. It may come updated with a new wiring connector to fix the old ones problem. If it does, you have to rewire the van following the directions.

Maybe in front of passenger rear wheel area, up underneath attached to frame?