12-469 - newer radio that is easy to hack, but makes an obnoxious click.  This radio is found on eBay for sale, but is not a great purchase.

20-125 - This is a good box, but is more difficult to hack. Steve Hultay discovered this one, too.

Purchase a Radio Shack 12-820 arm band radio and remove the four screws on the back.  Lifting the back cover off carefully, you will see two bundles of wires.  Both bundles should contain mainly gray wires, but with one red wire to the far right of each bundle. You are concerned with only the left bundle of
wires. You will need to carefully snip the third gray wire from the left of the red wire of this first bundle (do this at your own risk - see illustration below).  If you were to remove one more screw from the circuit board, you would be able to separate the circuit boards and see that this gray wire is the mute wire.  It will
cause your arm band radio to sweep without stopping when the up or down arrows are held down for about a second.  Your radio should still work and you can manually tune to any station; but now it should scan the FM or AM band without stopping.  In our testing, we have noticed that occasionally the sweeping of the band will stop for no apparent reason, but overall the hacked radio worked quite well for us.   (Also see: More Ghost Box Hacks!)

The next thing you will need is a set of speakers that are amplified to use with your hacked radio.  If the speakers take batteries, they are amplified.  Simply plug your pair of amplified speakers into the earphone jack to be able to hear the radio without earphones.  Your ghost box is complete.  For our model, we added velcro to attach our radio to the outside of the Radio Shack 40-1441 folding speakers (see ghost box photos below).  Our speakers cost $12.97 and the armband radio was $32.99.  If you do the hack correctly, your ghost box will cost you under $50. and will sweep both the AM or FM bands.  The radio is nice, as it also has a bass boost.