Light selector switch seems quirky. Also sounds like you could be shorting/grounding out, try to find the switched 12V contact on the light switch that causes the blown fuse and with the battery disconnected (and a new #3 fuse) check for continuity between ground and the 12V contact. Then you have isolated that it is shorting out. Post back and I'll see if I can find some good isolating troubleshooting points to test from there. Seems that because it was an intermittent problem at first then continuous, a loose wire shorting out is a rational possibility.