Check the fuse and work back from there. Also check the fuse for the ABS and braking. Also.. check the switch itself! Just pull it out and test the contacts. It's usually just something simple!
Yeah I'd actually say start at the switch. The more common failure would be for the ASR warning to come on all the time (indicating that the system is not working). Your system seems to be working fine and as far as we know it's only the manual disablement of the system that is causing a problem. I'd start at the switch. You may find a previous user disconnected the wires for fear of someone being tempted to disable while driving.
I cannot get my ASR to turn off with the switch. I checked the fuse behind the passenger seat. It's good. I pulled the switch and it has 5 contacts. I get continuity between 2 of them but nothing between the other contacts. Does anyone know how to test it????
My switch and light work AFAIK but I wonder if the system really does disable with ASR off. I have not spun the tires yet but it does seam to slip too much still. I had it fine tuned with PIS last month and it is very close with good specs. I get a weird hesitation when shifting to 2nd and 3rd below 5k rpms. Fine above that. I think VVT complicates things. Maybe also slight movement on pedal (TPS) from g forces.