The simplest form of reverse polarity protection uses the fuse and a diode.
A polarity reversal should forward-bias the diode (make it switch on and conduct) and cause the fuse to blow rendering the device safe from further damage, but in need of a fuse.
This very simple scheme works only if
1) The fuse blows fast enough
2) the diode switches fast enough
3) the rest of the components in the device that receive start-up power can withstand the initial surge.
Needless to say, there are better ways to do this.
BTW, you are supposed to be measuring for 12v from chassis to one side of the fuse, then the other.
If you read 12v across a fuse (that would be a "short") the fuse must be open (blowned)
Be careful, and don't get zapped by the P48 circuitry!
Best to do most benchwork with power off and disconnected.
If you get a replacement fuse, be absolutely sure that it has the same speed characteristics as the OEM part!