a simple solution to mount a seat post mounted light on your rack.
 

tail lights and accessories only mount on your seat post but if you have a bag or rack I the way nope. so I just used a cheap seat tube I had cut a section off drilled a big hole for a tool and a small hole for the bolt and put it on the back of my rack. I could also have used a self tapping screw if it did not already have a bolt.
https://cimg9.ibsrv.net/gimg/bikefor...ec095b6a7e.jpg
https://cimg7.ibsrv.net/gimg/bikefor...3a7c6871fc.jpg

If you have a standard rack like the one shown below you can mount a light on it with just 4 parts. Here is one that dates back a couple of years that will work with a variety of lights. I screwed the fixed part of the Raypal 2266 rear light permanently to the plastic tube but with this light you can remove the light from the tube to charge it.
plastic tube or thin-wall aluminum tube (1/2" schedule 40 PVC pipe or an old aluminum tent pole)
two nuts and bolts
one corner bracket (available at my local dollar store in a package of 6)

https://cimg6.ibsrv.net/gimg/bikefor...6c74f15248.jpg
Raypal 2266 USB rechargeable tail light

https://cimg0.ibsrv.net/gimg/bikefor...11e9f7b387.jpg
Light and tube separated

https://cimg3.ibsrv.net/gimg/bikefor...f0e3635247.png
where the L bracket is mounted to the rack
https://cimg6.ibsrv.net/gimg/bikefor...1798d2bfd4.png
Lezyne Drive clone mounted in the same way

Good idea. But I wonder if a piece of PVC pipe would work just as well, and not add that much weight to the bike.

That is PVC pipe in my picture. A 16" long section of Schedule 40 PVC irrigation pipe weighs 100 grams (~ 3.5 oz) and you only need about 2". The 2" steel corner bracket weighs 16 grams (0.57 ounce). The advantage of using PVC is that it is much easier to cut and drill holes in it. You can use almost any hard plastic tube that is 1/2 to about 5/8" diameter.

I no longer use lights that require disposable batteries now that USB rechargeable lights are available. The 100 lumen Raypal 2266 shown has a 500 mAh lithium polymer battery and the Lezyne clone has a 650 mAh lithium polymer battery. Both lights cost me less than $5 and will run for at least 6 hours in flashing mode.

good ideas. I discovered when I tried simplifying it just attaching the gamin mount the back of the rack the radar hits the fender. I had thought of a center post but my rack does not have a center platform if I had my bike at my work I could make something but this works well and keeps the radar away from metal.

B&M (Busch und Müller) (and other companies catering the european market) have some battery powered taillights which can be mounted directly to light mounts on racks. you could also look into e-bike rear lights for racks and connect them to a battery. And then there are rear lights which allow mounting on a back pack or saddle bag, so you could attach those as well to the rack. Or you get a mount to attach your light directly below the saddle. there are some adapter for saddle rails.

I cut some rubber strips from a blown inner tube to wrap around my rack to make it a bit thicker so that I could fit a seatstay light mount onto my rack tubing. When touring, I like to have two taillights in case one fails, had one fail from corrosion (too much rain) on one tour. Normally only use one but in fog I often run both.

https://cimg6.ibsrv.net/gimg/bikefor...adfc689072.jpg

After the photo was taken, I also added a reflector to the light bracket on the rack.

actually this is a e bike and I do have a twilight powered by the bike but I put it on the fender as that one was lighter. but the seat post is a suspension seat post so its not practical. plus the radar needs to be away from metal so this works well.

Well i didn't mean the seat post, but the saddle rails... so that should work with a suspension seat post as well. something like this https://www.lupine-shop.com/en/acces...t?number=i1099 But then a quick research showed that for the garmin there is no such special mount, so you'd need a gopro mount for the saddle tubes/rails and then a gopro adapter for the garmin, which is probably a pricey option...

this is a tandem and having a light under the back seat is harder to reach. this works well the radar has a clear view that it requires and I don't have to worry about bags getting in the way of a seat mounted light. a wired fender mounted taillight that's always on the garmin radar with a tail light and now a backup taillight that is controlled by the garmin.


https://cimg5.ibsrv.net/gimg/bikefor...985158c06e.jpg