Idiot Mistake...Dead Car!!

[bigjae1976]

So did you put a fuse on the +12v lead yet?

[hlmiii]

Originally Posted by bigjae1976

Seriously? That is DEAD WRONG!

It is NOT dead wrong.

A fuse is nothing but a pass-through link (in these applications). There is NOTHING in it that reacts to polarity.

Think about this: did you ever blow a fuse because it was inserted in the wrong direction?
No!
That's because there is no direction - no polarity.

Now ... reversing a circuit's polarity can cause a spike in current in some instances, when a polarity-sensitive device burns out and results in a short to ground. The high current can then blow a fuse.
See kouhii's post as an example.

Originally Posted by bigjae1976

If a bare +12v wire touches ground, the fuse blows.

That's because the short circuit creates a huge current draw: the reduced resistance results in a much higher current, which blows the meltable element in the fuse.

The polarity stays the same.