brake light sensor, i assume it is the brake light switch attach to the brake paddle?

Will, the best to diagnose your problem is to get a fault code reading and don't just rely on the suggestions, there are right and wrong way of approaching your problem, while many are helpful with their suggestion, not all ASR problem may be the same. (right Pete?)

the best and only proper solution to your problem is to first get the fault code, with out it your working blind. Your S600 is equipt with mostly electronic control modules, don't mess around with it, unless you want more headache. post the result and I'll try to guide you.