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 Hotshot Micro 2W USB Rechargeable Tail Light
Mirrycle Incredibell Original Bell
Busch & Müller IXON IQ LED Headlight (without Battery Charger and NiMH Batteries)