Mini Cabrio door lock issue

Sounds like a busted door switch to me. Have you checked the wiring harness where it enters the vehicle doors? Those can get brittle and crack over time. Any sign of corrosion on the door latch connector?

Where exactly is the door switch located? And how can I check the wiring harness for damage?

The door switch is integrated into the door latch. To check the wiring harness, peel back the rubber boot between the door and the body. Look for cracked or frayed wires. If you're not comfortable doing that, a workshop can easily diagnose it. Sounds like they will need to replace the door switch.

The workshop confirmed it was the door switch and a faulty wiring harness. They replaced it for 95€. All good now.