Probably the pressure release switch. On your car I believe it's on the bottom of the booster asmbly, and part of the brake controller system.

This release switch has to coordinate with the brake light switch on the brake pedal. If more than 50ms off, it will flag BAS. WIS will show the correct problem. If it's the release switch, I don't know of a fix other than module replacement. They can be had at junkyards if you know what you're after.

If you're talented with a diode scope, you can touch resolder the module TTL attach points to the original board and save $800

Note that newer MBs don't have a brake light switch at all, instead using pressure averages for computer controlled braking.

Note2 One time I replaced 4 brake light switches before I got a good one