There's a metal shield about 1cm/1cm (not a perfect square, but you get the idea). If you open it up and take the green board out, it's on the back, to the left of the metal prongs that touch the scanner battery.
Contrary to what I thought a few months ago, it seems to make no difference whether it's there or not. I've taken them out of a few devices and it doesn't seem to make a difference.
The only problems I've seen with any devices lately have been the iPod connecting to the sled. The battery seems fine, the iPod itself is fine, the sled is fine, and the software works great. It's the physical connection between the iPod and the sled that seems to suck balls. Removing the iPod from the sled and putting it back in can temporarily fix a lot of my issues. Then it scans for a bit, and then disconnects.
@mobileFellow