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.
CygoLite Rack Mount Bracket for Hotshot
Busch & Müller Ixon Pure Bike Headlight
Busch & Müller DToplight XS Senso Bike Tail Light