I can't help you with the hard wiring, but most people use CTEK tenders. However, any standard battery charger that charges AGM batteries will work with the M2/M2C. BMW branded battery chargers come from a few different companies, but their latest are from CTEK. I bought BMW's latest CTEK charger because it has dual AGM and lithium batter settings, and I couldn't find this on a CTEK branded battery charger. (At the time I had an F80 M3, which has a lithium battery, but I figure as more cars switch to lithium batteries it is nice to have a charger than can be used for both AGM and lithium batteries).
https://www.ecstuning.com/b-genuine-...r/61432408594/