the sound upgrade chip. The chip splits the frequencies and allows settings to be saved.
Remove the stereo, the info can be found
Locate the cover on the top of the stereo, boxed in red below.

Using a Torx10 bit...

remove the cover.

This is the smart sound upgrade chip.

It slides straight into the stereo until it is flush with the top, the screw hole will line up.

Refix the cover back over the top and screw it down.

You will now need to activate the sound chip which can be found at the bottom
The smart part number is Q 0001300V001000000.
However, after all this time, there is very little chance that you'll be able to buy one from smart.
Your only good option is to keep an eye on eBay, cross your fingers and make a wish. They are very rare.