If you use a relays you need a coil supressor (Diode or Resitor).
Good (expensive) relays have a build in coil suppressor usually a Diode, cheap ones don't.

If you open a switch in a 12 volt relays system the relay coil generates up to 1500 Volt, extremely bad for the car electronics. If the fuse blows it just tries to protect the electronics in your car. Replacing it with a lager fuse means something else will blow up, usually something much more expensive. It may take a while but it will happen.

The simplest form is a diode parallel to the coil (with the bar on the Plus side) That way the current passes trough the diode and not back into the system. This diode needs to be large enough to handle the current.

As for the light fuses, never use a fuse larger than needed, 12 volt, 100Watt = 8,3 Amp.
The fuse needs to be a little larger, the current trough a lamp is higher before it warms up.
If you want to calculate the maximum current check the lamp resistance, if cold it has the lowest resistance (Voltage= Current x Resistance).


Rob