This is an extremely simple and quick modification - nothing special but it improves the tuner's ergonomics IMHO.
The LED is connected to the coils of the relay responsible for switching the 160m inductor in series with the main roller inductor. Current limiting for the LED is done with a 1.8 kOhm - 2.2 kOhm resistor. The LED can also be connected straight to the 160m switch board but it is very hard to get to the board with a soldering iron without removing the top peak-hold/switch board.
The key component for the mod is a 3 mm LED in a small yet elegant black bezel - Digikey P/N 67-1192-ND. It looks excellent on the black front panel just above the 160m button. This LED bezel assembly requires a 4.1 - 4.3 mm hole. One needs to make sure that all of the metal particles from the drilling are completely removed from the enclosure!
Maintenance tip: I noticed that the roller shaft was very dry and needed lubrication. I applied very small amount of conductive grease using a fine brush. The grease needs to be brushed as a very thin film just on the shaft. (!) One must be very careful not to get any grease on the roller itself or the inductuor's coil as this might cause huge arcing problems. The tuner normally comes with factory applied conductive grease but on my unit it wasn't enough. I moved the roller up and down the shaft a few times to evenly spread the grease while making sure there is no excess that can fall onto the inductor.
For DIY conductive grease : I used some white lithium grease (sold as Genie Screw Drive Garage Door Opener GLU-3 lubricant - this grease doesn't gum up easily) and added a good amount of fine graphite powder (I used a fine file (! not sandpaper) and soft carpenter's pencil to produce the powder). I mixed it well, while adding more graphite powder until the mixture thickened a bit.
2010 Update: Palstar changed the design a bit for their current version of AT2K. They no longer are using the 18 uH inductor which had to be padded with extra 10uH inductor for the 160m band. The new inductor is 28 uH therefore there is no longer need for a relay and switch in order to add extra inductivity and tune 160m so this mod is irrelevant.