[Dmitry at Pelican Parts]

Hi Chuck and welcome the forum! I'm attaching a link to a DIY tech article from our site on the E9x platform; specifically where you can find access to the battery terminal cable. If you have any other questions, please feel free to post a Comment just below the article and one of our techs will get back to you with more details. Hope this helps you get the M3 back on the road, best of luck!

https://www.pelicanparts.com/BMW/techarticles/BMW-3-Series-E90/BASICS-Battery_Connection_Notes_Replacement/BASICS-Battery_Connection_Notes_Replacement.htm