I was wondering if you could recommend a tail light for me. I currently have a Tubus Disco rack with a Busch & Muller 4D Toplight. I just got a new pair of Ortlieb Back Roller Classic panniers, and now the light is too wide. The light pushes the bags outwards, making them difficult to put on and take off the rack. I'm looking for something with similar visibility to the Toplight, but a little narrower.

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There are a variety of Busch and Muller Taillights, however, they all have a similar profile and will likely cause similar issues on a narrow rack with panniers.  There are several other taillights that I would recommend that are both narrow and have available rack mounts.  

The Cygolite Hotshot offers great visibility and a rear rack mount is available.  Another direction to consider are Planet Bike Superflash Taillights which also have a rear rack mounting bracket available.