I did some research that I thought is worth passing on regarding fuses on both negative and positive leads for mobile ham installations and similar equipment.
First off the main reason to run both +/- power leads directly to the battery is to minimize the amount of 'noise' that the radio will pick up from the vehicles alternator, fuel pump, wipers, ect.)
They fuse the negative side because if you connect the radio DIRECTLY across the battery in a vehicle and something happens to "remove" the battery ground, such as a loose bolt, broken cable or lifting the battery ground (-) wire for vehicle service - The return current for the vehicles battery will go directly through the radio to power accessories, headlights or worse - the starter.
When placing fuses on both negative (-) and positive (+) leads, the fuses should be located as close to the battery as possible to offer protection for the entire wiring harness.
The concept of double fusing is not just made by some over reacting hams but is recognized by General Motors in their service bulletins:
http://service.gm.com/techlineinfo/radio.htmlOn a side note: Within the radio the negative lead often does not go directly to the radio chassis. It often enters to a printed circuit board like the power amplifier section. If the leads were reversed or voltage applied to the negative lead in reference to the radio chassis, it could damage components inside the radio.
