Ive got a '99 Audi a4 and recently the airbag warning light has come on and won't go away. The Audi dealer told me the wiring under the drivers seat is either worn or torn and a replacement would cost me at least $500. Is there a way I can sort it out without forking out a stash?
That price is for the dealership to replace/repair. It also covers their warranty, factory warrantied parts etc. - dealers are generally much happier about sorting problems with official parts.
You could take it to an independent, which will be cheaper, but you won't get the factory warranties. They should hopefully still provide a warranty on their work - ymmv.
Our you could do it yourself, which should be cheapest but obviously you will get no warranty. What is your risk appetite here? You can buy parts from a number of places - wrecker's yard, online oem parts stores or the manufacturer direct.
If you are confident doing this yourself, it is one of the simpler repairs, but basically you get what you pay for.