Ok, I have posted three "representative" wiring diagrams to help us walk through your issues. The first one shows the schematics of the actual stator coils. The upper RH coil is the one we're dealing with. The second shows pictorial representations of the electrics which can make it easier to understand what is what. The last is in colour which is much easier to trace than the top two.
First, there is no ground at the coil. Each side provides power to different items but the ground is provided by the other side.
Second, there is no regulator. Voltage is controlled by the load. High voltage means not enough load.
Third, the "blue" side feeds the brake light.
Fourth, the yellow side feeds the taillight, pilot light, headlight and, if equipped, the speedo light.
Fifth, notice each diagram lists specific bulb wattages.
Your problem is that the voltage is too high and the taillight bulb keeps blowing. If the voltage is too high, that means there is not enough load. The usual cause is that one of the bulbs is blown, or the wrong wattage is fitted. Check ALL your bulbs. make sure they work, and that they are the correct wattage. Don't forget the speedo light if you have one.
Thanks for the diagrams, I only had the basic wiring diagram before. Since my last post, i have ripped the entire wiring loom out. Glad I did as it was full of bodges, bare wires twisted together etc. Made my own loom, brake light and horn work, I think the individual stator coil that I fitted wasn't the correct one so if it ever stops raining I'm going to fit the original blue / yellow wire coil back into the stator. I will let you know what happens.