So I've replaced the fan motor and switch in my car, but I'm unable to get the defrost going, or redirect the airflow to the floor only..is this a vacuum line problem? How do I diagnose where the leak may be coming from? Thanks as always for the advice.

Nick

 

I would use a Mighty Vac to test the system for leaks. Assuming you have a 240 - you don't mention the model in your post, trace the vacuum from the engine all the way to the control flappers checking for leaks along the way. The vacuum that runs the HVAC system starts at the top of the intake manifold where there is a hard black/white check valve. From the check valve there is a small vacuum line that runs through the firewall in the passenger side footwell. From there it runs to a reservoir in the center console. There should be a separate vacuum line that runs from the reservoir to the controls. From the controls there should be individual lines that runs to each of the flappers.

I'd start at the line running from the firewall to the reservoir. Disconnect it from the reservoir and using the Mighty Vac, pull a vacuum on the line that runs to the engine. It should hold a vacuum. Keep moving towards the flappers until you find your leak

 

for the uninitiated, a MityVac is a handheld vacuum test pump, extremely useful for all sorts of auto diagnostics. I got this inexpensive kit, https://www.amazon.com/Mityvac-MV800.../dp/B00265M9SS and have found it useful for all sorts of things, from testing EGR to hvac stuff like here.