B Klasse cruise control issue

I had a similar issue on my B Klasse a while back. Check the brake light switch first; it's a common culprit. If that's not it, inspect the fuse box. A blown fuse can definitely disable the cruise control. The pedal switch could also be the issue, although less likely. By 'pedal switch', I mean the sensor that detects if you're pressing the accelerator or brake. It's all interconnected for safety.

Where exactly is the brake light switch located? Is it easy to access, and can I test it with a multimeter, or is it better to just replace it?

It's usually located near the brake pedal, activated when you press the pedal. Access depends on your car's configuration; you might need to contort a bit. You can test it with a multimeter, checking for continuity when the pedal is pressed and released. Honestly, unless you're comfortable with electrical testing, it might be simpler and safer to let a workshop handle it. Considering the initial diagnosis pointed to a cable/plug issue, they'll likely check all the connections thoroughly anyway.

I took it to the workshop. It was indeed a defective plug connection, as initially suspected. The repair cost 95€, cruise control is working again.