Additional Battery Alarm System Defective

Strange issue you're having there. I had something similar with my Mini Clubman – the alarm would out of nowhere go off. Have you checked the auxiliary battery? A weak auxiliary battery can cause all sorts of electrical issues. I suggest running a battery check, maybe at a workshop if you don't have the equipment. It's worth investigating before you start tearing apart the alarm system itself.

That's interesting about your Clubman. When you say 'similar', did you also get an error message about the alarm system, or was it just random alarms? Also, did a system diagnosis point directly to the auxiliary battery, or did you find that out some other way?

Yeah, I got some weird electrical warnings too before it finally went belly up. A system diagnosis confirmed it was the auxiliary battery. Honestly, I'd avoid messing with it yourself if you're not comfortable. Just take your Cabrio to a workshop and let them do a battery check; it’ll save you a headache in the long run.

Alright, just wanted to give you a quick update. Took the Cabrio to get a system diagnosis at a workshop. Turns out the auxiliary battery was indeed the culprit, just like you suggested. They ran a battery check, confirmed it was failing, and replaced it. Alarm system's back in action. Total bill came to around 330€. Thanks again for the advice; really appreciate it!