Is the fuse the right rating?
If it is, you have something that is drawing too much power. If it blows the fuse as soon as you turn on the switch, it is a short somewhere in the wiring.
If the fuse is glowing red, the amps being pulled though the fuse is slightly over the rating of the fuse.
And yes, fuses can glow red without blowing at least for a while. I have seen it happen many times.