When you use your blinker light your backup light also flashes how can you solve this?

Backup Light Flashes "With" the turn Signal Lights

This should not be happening. I suspect that someone has been messing around with the wiring for the rear of the vehicle and has "screwed" it up.

IF, for example, someone who didn't know what they were doing tried to add a connector plug harness for trailer lights, and made an improper connection, the circuits for the turn signals, and the backup lights would be interconnected.

This would result in the backup lights flashing in tandem with the turn signals.

Remove the inadvertent "bridge" between the two totally different circuits, and the problem should go away.