Mercedes SL Software Issue

Sounds like a potential Vehicle software bug. When you say 'implausible programming', that often means the control unit (the car's computer) has corrupted software or incorrect data. The navigation system failing alongside other electrical issues does point towards a central software problem. Has the car had any recent work done, like a new battery or any module replacements? A sudden power surge or drop during those procedures can sometimes scramble the software. A software refresh could indeed be the fix, but it needs to be done properly to avoid further issues.

Thank you for your reply. No, no recent work has been done on the car at all. So what exactly does a 'software refresh' involve in this case? Is it something I could potentially do myself, or is it definitely a workshop job?

Okay, given no recent work, it does sound more like a spontaneous software malfunction. A software refresh for the control unit essentially means overwriting the existing software with a new, hopefully bug-free, version. This is often done using specialized diagnostic equipment that connects to the car's OBD port (on-board diagnostics). While technically possible to do yourself if you have the right tools and software, it's HIGHLY recommended to have it done at a qualified workshop. Incorrect flashing can brick the control unit, making the problem much worse and far more expensive to fix. Those 240€ might be the best money you spend!

Thanks again for the advice! I suspected as much. I'll get it booked in at the workshop. They confirmed they'll do a Vehicle software refresh on the control unit. Hopefully that will resolve the software malfunction. It's a relief to know it's likely just a software bug and not something more serious.