The T5 has a very simple 12 volt AC electrical system but one thing may not be clear when you look at the wiring diagram.
It appears that the output from the alternator goes firstly through the horn, this does not make much sense. If you are fault finding it is important that you understand how the system works.
Why is this unusual system used?
To ensure that all the current is available for the horn when it is operated. On any circuit the horn may not sound efficiently if it is supplied with low power.