I'm trying to spruce up my daughters 91 teg and have a question about the defogger. Either it is not working properly or I'm too stupid to figure out how to use it

This is what I do to try to get it to work. I turn on the A/C, turn the switch to outside air, then I press the defogger button on the unit. I see a light come on temporarily then it goes off and virtually no air is pushed out of the windshield vents.

However, when I switch to combination (where part air is coming out of the bottom vents and part is coming out of the windshield vents) it appears to work. Unfortunately sometimes you need a full blast on the windshield to keep from fogging up.

Anyone have any pointers? Am I doing something wrong or do I need to a new heater unit? Thanks for the help.