Taillight fuse pops every time I turn on my headlights. Can't figure it out

Check your tail lights and running llights for damage, particularly the sockets.
Disconnect tail light wire going to the back to narrow down.
Any trailer wiring?
Recent wiring or after market wiring?

Ya I put in a new stereo. But I thought I covered all the loose ends. Someone mentioned the ignition switch. That sound likely?

You didn't say what vehicle you have. Tail lights usually don't go through Ignition switch.
Did it happen right after stero installation?