My 66 fastback did the same thing before I tore everything down and then the problem disappeared. There are a couple of places that can cause the fused courtesy lines (green/yellow) to blow when the courtesy lights (black/blue) are activated by the door jamb switches or headlight switch. I'd start by looking closely at the door jamb switches, as it is very easy to short out the plugs into the switches. The other common area may be the rear sail panel lights, as the lines may be pinched at the door sill or as the wires go over the wheel wells.
A good trouble-shooting procedure is to remove all courtesy light bulbs and see if the fuse still blows. It probably will. Then disassemble on door jamb switch, being careful with the plugs to keep them from shorting. Then try the other door. Still blowing fuse? Disassemble the other door jamb, and then try courtesy lights with the headlight switch. Still blowing? Then somewhere a wire is pinched.