Mount a tail light to your rear rack. If your rack has no rear hole for mounting a tail light, but does have either a top plate or some cross braces, clamp this T bracket your rack.
If your rear rack has a single mounting hole, use the Busch & Müller Tail Light Bracket For Single Hole On Rear Rack.
Planet Bike Superflash Rear Light
Busch & Müller Tail Light Bracket For Seat Post Binder
Busch & Müller Fork Crown Mount for IXON Speed and other Bike Headlights