[ChazzyD]

If you disconnect an airbag wire (like the ground wire) and turn the ignition on, the airbag computer will get upset and turn the light on. I hope you didn't do something like that because now you will need the Marelli tester to reset the light (actually you already do).
Charles

 

[ChazzyD]

I had that exact problem when my A/C stopped working last year: a loose or dirty wire connection at the frost switch. Remove the small black plastic cover and clean the connections to see if that helps. Mine started working again once the contacts were cleaned up. I still had pressure and couldn't figure out what happened until Alfisto Steve nailed the cause of the problem, once again.
Charles