Pictures are clickable for bigger versions.
The type of gear indicator that you can buy anywhere, calculates the gear from the revs and the speed. This requires some fiddling to start with and it doesn't work when you have the clutch in. For some bikes, the TRX being one of them, this can be done much easier and better.
The neutral switch is a small disc that makes a connection when the gearbox is in neutral. Every gear has a different position on this disc. So if you make more connections, one for every gear, you can actually measure in which gear the gearbox is in.
The pictures below show the original TRX neutral switch, with one connection. The wear track indicates where the other gears are located.
You can make the neccessary connections yourself, if you have the courage to drill in the neutral switch. Small nails can be used to make the connections. This looks like this.
There are also neutral switches that already have all the connections. Coincidently my YZF had one like that.
The neutral switch of the YZF and the TRX are exactly the same. My TRX has a sensor with connections for 6 gears... It took some time but I found a part number of this switch: 4TX-82540-40 (from a TDM, '96 and later), but it's not cheap, almost 100 euros.
The neutral switch is located behind the front sprocket. In fact it is a switch that makes a connection to 'ground' / - and makes the neutral indicator lamp to light up. To show the gears there is a 7-segment digit display needed, which is nothing more then 7 horizontal and vertical orientated LED's. Each gear has a connection from the neutral switch to the display. Realise that when the bikes is in neutral and the "0" is lighted, the 0 is formed with 6 segments. With the "1" only two segments are lighted. Etc. A diode is needed for each gear and each digit segment. So, that is a load of diodes. Besides that, 7 resistors are needed to limit the current trough to all segments of the display. The resistor also determines the brightness of the display. Unfortunately, I don't have an electrical diagram anymore. The only thing I have is this photo where the diagram is vaguely visible. What I do know is that the display has to be of the type with a common anode.
When you solder this together in the right way, you get this:
Then you have to find a place on your bike to put this box of course. Besides the neutral switch and the display, it is connected to +, preferably one that switches on and off with the ignition key.
It took some time to get it there, but the display is nicely fitted between the speedo and rev counter.
Because this trick works for several bikes, there are different descriptions on the net. Some people used IC's (microchips) in stead of the diodes, but I am not into that kind of stuff.
See for an example here (in German, sorry, but there is a diagram):
http://www.alpentourer.de/shop/ganganze ... zeige.html
There is hope for the lazy ones, there are gear indicators for sale that work with the same principle, see http://www.ledgear.co.uk.
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I translated this from Dutch, please let me know if phrases could be improved or clarified.
I was following nicely right up to the picture with loads of fiddly bits in it now my head hurts. 
