97' Jeep Wrangler- Very little floor heat.

Hello Everyone- I discovered that the floor heat on my 97 Wrangler does not blow as much as it should. I have narrowed it down to the yellow vacuum line on the floor defrost mixer actuator is not pulling a vacuum when selected to floor only heat. The problem is that I do not know where the vacuum source comes from in this floor only position. Let me know if you can help. Thank You!!!

  jebrooks4