Hopefully i can help..
There could be many reason for the airbag/srs warning to appear..it goes through a self test each time you first start to use the car in which once all self is complete and no faults are found by the airbags systems own ecu the warning goes out..obviously if it stays on then a fault has been detected..you really can go replacing parts of the system without proper diagnoses and that has to be done diagnostically before you even touch it..diy'ers arent reccommended to play around with the system as each airbag is an explosive device in theory,and any old airbag must be disposed of correctly..Usually the most common causes can be due to plug connectors,wiring in particular under the front seats or connections to the airbags are faulty..unless the car sees one of the bags in itself faulty..either by a wiring problem from connector to the bag or air bag igniter...I do hope this helps
How would one test the wiring up to the point just before the bag so as to identify whether the bag was faulty or that the wiring itself was the cause of the problem
Are there any fuses or resistors in line that may have gone?
I would not advise using a multimeter if that was intented to be use for checking/resistance of a defective airbag,igniter...something like an air bag load tool is needed to simulate an air bag to the computer,diagnsotic machine to test the wiring circuits,this is to prevent any shorting and accidental deployment of the air bag itself that why i wouldnt recommend for anyone to test without the proper experience and equipment..I cant see it being a fuse nor resisitors.You dont say what specific testing has been carried out diagnostically as this would give a much more detailed insight as to what were are,you dealing with here..it might not nessesarily be a specific airbag at fault it could be the actual airbag control module or seat belt pretenioners or even loose plug connecter that just needs to be pushed fully home.not forgetting of course you havent got the passenger airbag deactivation switch to off,often that can be over looked something to rule out...sometimes that can often happen..only way to know that would be to retrive the faults codes and see if any of them relate to a open circuit on a specific area of the airbag system and check accordingly befoe which ensuring the battery has been disconnected and the system given upto 15mins before checking that as per tracing the wiring back from the airbag to the control module.ecu..