do you know how to pull the TEST mode in the cluster? there are many posts floating on this board - search for hidden menus, check the HOW-TO section... if i remember right, TEST6 is the one that pulls the onboard voltage... when car is running, the voltage should be around 13-14V, ocassional dip to 12V and maybe ocassional spike to 15V... anything below or above those dips and spikes - issue with battery and/or alternator... a dying battery can trick the GM into many things... and the battery can have bad cells, which the autozone or whatever the battery place can check for free, and if you have the warranty that is over 5 years, they can prorate you on the new battery...

regardless, it is a good idea to start the electrical troubleshooting with the battery... if the battery is good, then move onto the GM... if you have a BMW scanning software/tool, you might be able to interrogate the GM to see if there are any internal errors stored there - they would not set off the check engine light, as they have nothing to do with the engine, like burned sunroof motor or whatever... a really faulty GM modulemay not show errors, as the error reporting might be compromised but you are trying to make an informed decision, so more information you can take in, more informed your decision will be...