The nano iDSD BL receives digital, only. Most iPods do analogue out, and there are only a certain generation of iPods that can do digital out.
You can find out if your iPod has a digital output via this site: https://files.freemyipod.org/~user890104/bootloader-ipodclassic.html
Just check the model number on your iPod.
If your iPod can do digital out, you may need to ‘jail break’ via ‘Rockbox’. However, we can’t advise this as it could brick your device. If you wish to try, it would be at your own risk. See link here: https://www.rockbox.org
If you do this, you must make sure it is an official Apple Camera Connection Kit. See link here: