Tama naman wiring mo ... other than the overrated fuses.

Automotive relay coils usually need less than 1A, so 10A is an overkill. I don't have a Bosch SPDT right now, but a Bosch SPST needs only 150mA. With a 10A fuse the coil may fail without the fuse popping.

The Nautilus is rated at 18A, so a 20-25A fuse might be more suitable than a 30A fuse.

As for your fog lights, a pair of 55w lamps will need just about a 10A fuse.

Placing a diode across the relay coil might help, but AFAIK it only prevents any spike produced by the coil itself when turned off from going back to any semiconductor circuit and damaging it. You only have a mechanical switch ...

Moisture and heat usually cause a relay to fail, besides overvoltage and current overload. Make sure all connections are tight and soldered, if possible.

HTH