Well if SRS wires were cut / spliced, THAT would certainly be the problem. Some wires are "shielded" meaning they are protected from outside electrical noise. Cutting into that sort of wire could cause trouble.

Also SRS wiring might be totally monitored for resistance or capacitance to be DARN sure it has not been messed with and will work in an accident. That wiring might need to be replaced if someone tapped into it to connect a radio or whatever.

More modern cars have high speed computer networks running over special wiring. Cutting and splicing one of those wires can cause all sorts of trouble!

I would advise taking it to a factory authorized repair facility - be honest and say someone tapped into the SRS wiring.

Same with selling the vehicle. Tell them the SRS system has been modified and is not working. I would make them sign a paper stating they were advised of that - protects you should they be in an accident and the air bags do not deploy - someone gets killed.