The fuse blowing could have been as simple as you moving the wire loom around to see what all was under the dash to a more serious problem like frayed insulation.
Unless you blow the fuse almost immediately again I wouldn't worry about it.
If you do blow the fuse almost immediately you will need to start going through the circuit to see where the problem originates.
My first guess would be a problem with the ignition switch.
Every Type 'C' bus has problems with moisture under the dash. No matter how well they build them, no body is ever going to be completely leak free in the joint between the cowl and the bus body. The two parts flex and move at different rates so the gummy putty that seals the joint will leak. I had one bus where the leak dripped down onto the headlight switch. On another it dripped down onto the ignition switch. On both buses I had to R&R the switches and fix the leak.
My second guess would be some insulation has been rubbed off in one of the places where the wire loom rests against a piece of the body.
Good luck and keep us posted.