Cruise control issues

I’ve got a 1999 1.8L Passat and am having cruise control problems. Initially it was intermittent but now is completely inoperable. Along w/ this, my center brake light (in the rear window) stays on continually when the car is running – but only when my lights are on. When my lights are off all the brake lights seem to be fine. Correct me if I’m wrong, but the center brake light shouldn’t be on except when the brake itself is on?? I’m assuming that the two problems are related, but please once again correct me if I’m wrong.

Looking through various posts it would appear to be one of three problems: 1) Vacuum vent valve, 2) Cruise control module, or 3) Brake switch. Which of these – if any – would you recommend looking at first? Is there a good way to diagnose these three?