Originally Posted by Diseasel300

Assuming this is your 1990, pull out the glovebox liner or passenger airbag, whichever is fitted. You should be able to observe the flap operating in the rear part of the dash through the hole. Turn the A/C temp wheel all the way cold and hit the recirculation switch on the dash, you should see it fully close the outside air portion of the flap (100% cabin air).

Be aware that there is a switchover valve block around the knee area in the center console on the passenger side. The solder breaks internally where the individual solenoids are mounted to the printed circuit board. Cut the back off with a hot knife and resolder everything inside. I've redone close to a dozen of them and every single one had broken joints. It's also very common for the rubber elbows that connect the vacuum lines to said switchover valve block to split and leak vacuum.