Busch + Müller (Germany) make a variety of lights, including ones that mount on the rear of a rack.
Battery powered:
https://www.bumm.de/en/products/akku...cklichter.html
Dynamo generator powered:
https://www.bumm.de/en/products/dynamo-rucklichter.html
E-Bike powered:
https://www.bumm.de/en/products/e-bi...cklichter.html
Myself, I've got a pair of CygoLite Hypershot 350 USB. CygoLite makes a rack mount adapter, if that's your preferred mounting spot. Very bright light. Good range of flash patterns. With two of them, one on the rack and one on your helmet, seat post or seat stay, it's all but impossible to not easily see a bike from a few hundred yards away.
https://cygolite.com/product/hypershot-350-usb/
https://cygolite.square.site/product...&category_id=4