While I applaud his ingenuity, there already is a kit out there to do BMW sequential tails (on LCI tails ONLY)
Guy who makes them posted the kit info on the forum when they 1st came out - I have them & they work fine. PnP in about 5sec after u remove the outer tails - no circuit board construction needed - it's all contained in a short harness that goes between light and factory wiring & uses all factory connectors
Google BMW Dynamic Blinker Mod
Pic of harnesses on their site not quite what I got - mine look exactly like in utube vid
https://www.youtube.com/watch?v=Sge8PmrDwyA