where is the airbag monitor located on body style

where is the airbag monitor located on a 2004 f150 old body style

I'm not sure what you are trying to repair, but the location of the RCM (Restraints Control Module) is on the tunnel. I've attached a picture for your reference. Ford stopped calling them 'monitors' @ 1999.
There are specific items that are required replacement items when a deployment occurs or when an airbag light comes on...and there are some items that are "check only". Check out this website for the EXACT answers to your questions including fault code charts and reset information. This site tells you exactly what needs to be replaced and how to access them (seat belts & all) on over 3500 cars. It's a pay site ($14.95 / day), but it's little money well spent to get the correct answers and to see exactly what you're after...it uses pictures, not text, so you can see what you’re looking for.
This is the same website insurance companies and body shops use to determine what needs to be replaced.