Substitution of a cyclic diether by a halogen

[![enter image description here](https://i.stack.imgur.com/2GhWy.png)](https://i.stack.imgur.com/2GhWy.png)


I’m in an organic synthesis class and I have to draw a mechanism for the reaction in the picture. At first I thought it was just forming a free radical but I haven’t been able to find any way to do that without UV light, which isn’t shown anywhere in the given reaction. How do I go about solving this?


