AC panel issue

[97tbird]

Hello, new to the form and I have a 97 Thunderbird that just started to only blow the AC out of the floor and defroster when I have it set for AC to come out of the dash. I assumed it was a vacuum leak and looked under the hood, it appears that the vacuum lines under the hood are fine as I can feel the suction where the hose splits between the floor line and the dash line. So I'm assuming that I have a vacuum leak somewhere under the dash. Can anyone help with instructions on finding that leak or should I just purchase a repair guide that might have directions on how to make the repair.

Thanks in advance for any assistance you can provide.