Hi, I am doing a cafe racer rebuild, using a gps speedo and some basic idiot lights. One issue that is taking too much time is my tail light, it is an aftermarket light and have tested it and works well but seems wire from the "under seat" box is dead. Where is best place to run a fresh wire from? Ok if tail light is permanently on. Brake lights working fine

Thanks in advance

There is a fault there that you would be better to fix rather than just a work-around.
The tail light wiring is known to short as it passes over the rear shock top mounting.
#2 fuse may be blown.
Parking lights are normally always on.

You need to put some info up about your model and year as there are some subtle differences.

Thanks Holister, sound advise. Bike is a 84 K100 RS, went over it again, am getting 14 volts at the bulb for brake lights but only 7 volts for tail light, getting 7 volts at fuse no 2 as well.
Any thoughts?
Thanks 
Phil

Are you taking those measurements with the bulbs removed?

Sounds like the Bulb Monitoring Unit (BMU) may be dud. It can be removed but you need to jumper some of the connections to provide continuity to the tail and brake.
The BMU is on the left side of the electrical box under the tank.

Hi Holister, my BMU looks quite different from the pic (84 K100 RS), will post pic tomorrow, looks more like one side of the instrument cluster connector, is a Bosch unit with green cover if that helps.
Ta
Phil

Phil63 wrote: looks more like one side of the instrument cluster connector, is a Bosch unit with green cover if that helps.

That's the turn signal relay.......BMU you find on the LHS in the relay box.

Holister wrote:Are you taking those measurements with the bulbs removed?

Sounds like the Bulb Monitoring Unit (BMU) may be dud. It can be removed but you need to jumper some of the connections to provide continuity to the tail and brake.
The BMU is on the left side of the electrical box under the tank.

Or you can cut the guts out of the old one and make a "fake" BMU:

Hi guys,
Thanks for you help so far. Tried the BMU fix but to no avail, appears that I am not getting power to the BMU or the fuse box. Tail light works fine and all wiring from connector on seat rail to light is working well. I am thinking " what is the best wire to run for power to the fuse box and then on to the light, by passing the dead wire?" Unless there is something else I am missing.
Ta
Phil

If you're measuring 7V on the Grey wire at Fuse#2, then you have to troubleshoot further upstream to find out where the problem lies. It could be inside the ignition switch, or inside the light switch on the right hand cluster where a poor contact is causing a voltage drop. Solve that problem before trying to work around it with an undocumented wiring kludge (which you will forget in 2 years and then have to re-learn when another problem shows up because you didn't address the first problem in the first place).

Cheers, will have another crack.

Hi Robmack,
Will do some further checks, taking apart the right hand switch cluster will I be met with an explosion of little springs? Will do it inside a bag.

I would be inclined to start with cleaning the connectors for the handlebar switch blocks with DeoxiT®. They're found under the tank, one on either side. The Trouble Shooting page off the Portal Tab has a diagram of connectors. While you're at it, clean all the other connectors you can find as well as the earth connections. You'll find them under tank and gearbox. There's an earth wire connected to the right switch block also which might be where your problem is. Corrosion on that sw.block is common.

As robmack pointed out, the ignition switch is also a problem area. There are three power circuits switched thru that switch, one being the running lights.
Ignition switch link
Once you get this out of the dash and separate the switch from the lock, you need to be careful how you open it up and where the parts fit.