Sometimes you can find diagrams in some to parts catalogues.

The easiest fix is finding something recently disconnected under the hood (this is likely if it happend all of a sudden rather than gradully getting worse and worse). The next place is to pull up the floor mats for the driver and begin testing lines with the vacuum pump.

If you don't find anything, pull up the floor mats on the front passenger side.

There are two vacuum tubes for each valve... one to open and one to close. The lines are color coded for the open/close

It is not hard, just a real pain trying to track it down. Then usually taking off the door panel to access the affected valve. Definately a DIY. Just be patient.