In a 2007 M35x, does the alternator have an internal or external regulator? Car will start and charge 14.4v but after a few minutes the battery light and brake light come on simultaneously hence shutting off the alternator all together battery goes to straight 12.7v. And doesn’t go back to charging Unless I shut car off and restart. I have replaced alternator and battery.

I guess maybe start with the basics. Make sure every connection and the battery are securely fastened. You probably already did this. Next I would examine every wire and fuse link for wear and possible shorts. Then check the fuse link to make sure its seated snug and not shorting out.