2004 Mercury Sable airbag code 1-8

Not sure what the 1-8 code is. A big red flag would be your inclusion that the previous owner replaced the stereo. Many manufacturers have the airbag connections in close proximity to the stereo wiring harness. Was the CD player installed using a professional installation harness or did they just splice and dice wires? Were any other components added such as an amplifier? Was this issue present when you bought the car or did it occur afterwards. What were the conditions when it happened? Were you messing around under the dash? Did you check the fuses? When something is changed from OEM theres a lot that could be going on there. Airbags are important - perhaps taking it back to factory, checking the fuses and resetting the computer by unhooking the battery ground could do it or narrow it down. Good luck.

I am mot sure what the previous owner did but the the stereo wires have a lot of tapes on, and the airbag light was on before I bought the car. Maybe a bad decision, but anyway, unhook the battery does not work and I think I will check it in a professional shop, instead of messing around dozens of wires under the dash. Thank you very much for your replay!
YL