Hi. Where would be the best place to take the power for GPS, phone etc? I have a device pretty much like this one, that I want to put in the box under the seat. Then take the power from there to the GPS. I only want power on, when ignition is on. How do I do?

I would remove the crash pad which houses the ignition switch and find the wire which is live when the ignition is on (light green on my 1991 K100LT).  Run a wire back under the tank to the area under the seat and use that to switch a relay.  The power for the relay would best be from a wire with an inline fuse direct from the + side of the battery.  Use heavy guage wire to take a large current with minimal voltage loss.  You will then have an ignition-controlled source to power your GPS via the sockets, and you will also have a source of power for heated clothing, heated grips, etc. that you may wish to fit later.

xpc has the right idea
although no need to get into the switch cabling near the crash pad  

just pick a wire that is "switched " at the fuses in the electrics box ....under the tank

take that to the relay as a  switch for the relay , then pick off the starter relay terminals (battery side ) with a fused wire to the relay ...then the relay output wire to the sockets you are going to install  
there is an interactive wiring diagram link on the portal page which would give you the right colours ...and termination locations

In the relay box is a round white connector which may or may not be used but you will find the correct wire there.

Thanks guys, I'm gonna try that.

There is a white four-pin connector in the relay box. This has both switched and fused as well as unswitched positive and a ground. This was intended for a theft avoidance system, but is not used.

I also have a model to cast a mount for the gps, which sits in the middle of the handlebars, next to the switches. If you want pics, send me a mail address.

Den er ret nem at lave og virker fint.

Agerbundsen wrote:There is a white four-pin connector in the relay box. This has both switched and fused as well as unswitched positive and a ground. This was intended for a theft avoidance system, but is not used.

I also have a model to cast a mount for the gps, which sits in the middle of the handlebars, next to the switches. If you want pics, send me a mail address.

Den er ret nem at lave og virker fint.

PM sent. Soundt quite easy to make, think I'll give it a shot next week. Having a 230 km ride later today. Back sunday..