You will need:
Small Philips screw driver.
A red LED forward voltage approx 1.5V (or signal diode if you don't want it as dim, forward voltage approx 0.6V)
Multimeter (or try the LED both ways round)
Step 1: Remove the Bottom
Ensure you have unpluged the unit from the mains supply before you take it apart.
Note the two screws are hidden by the cables at the back of the unit don't forget these (Like I did) as it won't come apart untill they are all removed.
Once all the screws have been removed gently pull the back off. There will be some resistance and there are two cables still attached to the back (the antenna cable and a combined multiway cable to the power inlet and the USB) so don't pull two hard.
Step 2: Find the Right Wires
You don't need to pull the antenna cable all the way out and I didn't as it looked like it would be difficult to re-thread. You might even be able to get away without unplugging the multiway cable.
Identify the two loose wires shown on the right hand side of the unit in the photo. These go from the small PCB at the back with the brightness switch on it right the way to the front and connect to the LCD light. They are seperate red and black wires. All the other wires are ribbon cables of several wires connected together.
You need access to the red wire only.
They are held by hot melt glue and you will need to prise the wire out of this to make it easier to solder to the wire. If you are really good at soldering you may be able to do it without doing this.( I had pulled all the cables loose by this point as I investigated the best way to do this mod. Don't do this as the cables will rattle if they are not held down.)
Step 3: Identify Terminals on LED
To do this test it with a Multimeter. I used my trusty old fluke. You can see the forward voltage is 1.583 and the LED has light up. (although not all that brightly)
Note which side of the LED is connected to the red terminal this is the one you want to wire to the positive.
Step 4: Solder in the LED
Solder in the LED with the positive terminal (the one conected to the red lead) to the back of the unit. Trim the leads if required.
You can see my soldering is rubbish sorry about that.
I would test the unit at this point.
So plug the connector from the bottom of the unit back in and plug the unit in. The LED should light up and the display should come on fairly dim. It takes a few seconds for the display to initalise so don't panic if it doesnt come on straight away.
If all is ok. put some insulating tape arround the leads on the LED and cover the LED up with tape. They don't get hot so this will be OK. (you don't really want a red light shining out of the box)
If it doesn't work. Check carefully that the display is not really really dim and check the switch on the back, set it to the bright setting. If it is really not working don't panic the LED is probably the wrong way round.
Turn it off unplug it. Desolder the LED and turn it round. Try again it should now be working.
If not then maybe your LED is not working. To check this; unplug the unit, desolder the LED and remove it, then solder the wires back together. Retest the unit should now be working as it did origonally.
If not make sure you havent pulled any cables loose that the multiway connector from the PSU is firmly home. Once you have found out what is wrong go back and try again.
Step 5: Put It All Back Together
Once it is all working correctly. Ensure that the LED is well insulated and the light wont leak out with insulation tape. Then stick the red wire down with insulation tape so it doesn't rattle inside the unit.
Re-assemble the unit. Remeber to re-attach the multiway connector and gently pull the antenna wire back though. The body has some resistance, so you need to push it back together firmly before replacing all the screws.
The photo shows the completed modification in dim mode. In the light the photo was take in it is not that readable but in a dark room there is more than enough light.
All the modes still work but they are all much dimmer but I find it quite readable in bright light in the automatic mode and also OK at night. If you find it is too dim repeat the procedure but use a normal diode instead of a RED led. This will have a forward voltage of about half that of the red LED. It is very unlikely you will want to go dimmer but if you do you could use a red LED and a diode. A green LEDs forward voltage is so high(2.8v) you wouldn't be able to read the display, so you can't substitute one of these.