Actually now I'm not so sure what I wrote is correct. Most of the hard rules I find easily searching on StVZO are about front white light beam shape (which is a good thing to regulate TBH). Some articles mention flashing lights (front and/or rear) allowed during daylight but not permitted at night, but it's hard to tell what's really up to date. Not going to try searching the actual StVZO code (which covers the entire transport industry). I haven't yet found a specific upper limit to taillight brightness as a rule - just flashing. (I love my daylight-visible flashers but I always switch to constant-on as dusk approaches.) Getting way off-topic here though, sorry.