In reply to:

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If you take for example the horn: On some cars both pins
have +12V (measured to the car's ground and the car's
battery ground) and if you use the horn then on of the +12V
is taken to 0V so that the horn blows.

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Well, not that strange when you consider what they usually
do at the steering wheel end...

The horns is often wired: +12V to fuse block, then to horn+,
horn- to steering wheel switch which when activated connects
to ground. This simplifies wiring in the steering wheel; the
alternative is to go +12 to fuse, fuse to wheel switch,
wheel switch to horn+, horn- to ground - one extra wire to
the steering wheel...

The reason you measure +12 on both terminals of the horn is
that the negative terminal is floating, and unless your volt
meter draws alot of current (they're designed to draw as
little as possible), the voltage drop across the horn is
negligable. Instead all the voltage ends up across the
(very) high resistance volt meter...