I mounted my old rear light (a B+M [my:] E) on a little braze-on sticking out from my rear rack. This was a poor choice and it got bashed around. The cap came off the front of it and Busch + Müller does not sell a replacement. I rode it around like this for a while, including in the rain. Credit to B+M, it worked well even after getting wet.
I wanted the new one to be higher on the bike so I ordered a seatpost-mounted SON in blue with a clear lens from Peter White. So far so good! I really like the way it mounts and tensions to the seatpost. There’s a seatpost nut that sits in a recess in the housing and it acts like a dial. You rotate it until the tensioning strap comes out and then put it on your seatpost. Tool free!
I used Shimano Di2 cable guides to keep the wire neat down the seatpost. I really liked this approach. The SON coax fits great in the guide. One mistake I made was trimming down a longer section since the default length was a bit tight on the seatpost. The problem is the adhesive part of the guide extends past the cable part. This helps it stay adhered when the tension of the cable is pulling against the ends of the guide. Oh well. It still seems solid!
The wire for rear light power exists on the non-drive side chainstay close to the chainstay bridge. I left the wire a little long in case I adjust the light or seatpost height. I secured it with a zip-tie and the old fender strut clamp I used to have on the rear rack eyelet. This worked well since there’s similar retention for the rear brake hose.
It’s brighter than the old [my:] light but probably not as bright as a Secuzed or Topline. Photos are kind of useless at showing absolute brightness but maybe one day I’ll update this post with a comparison photo.