Odd E46 M3 Issue

[Carfiend]

Story goes like so. A friends M3 was back at the dealer as there was a misfire. This was fixed in the end needing a new sparkplug and coil pack. However 3 days after that the battery died the cause being the rear brake lights staying on after they brake pedal is depressed even after the car had been switched off.

This all seems rather odd however when the car was returned the boot battery cover was removed but it was still the same battery. So consider that it took them 3 days to diagnose a sparkplug and coilpack with all the on off engine running to check for a misfire I think they have charged the battery from the boot rather than the terminals under the bonnet which according to BMW can cause electrical issues such as the light controller playing up.

They have spent all of today looking at it and cannot find a fault. All the usual things like fuses, relays and switches have been checked leaving one of two no very cheap at all computers that are bust.

So considering the rear battery tray being removed, the nature of the previous fault that might of drained the battery and BMW themselves saying not to charge from the battery directly as it can cause electrical issues am I being paranoid to think the battery charge and the brake light problem are related?

[Carfiend]

Turns out it was the computers. They are being reprogrammed now and if that doesn't help then they unit itself will need to be replaced.