I guess one route would be a process of elimination. Start with ignition on, and everything else off, Then switch on components one by one until the ffuse blows, and then the last item you switched on could be the culprit. It's also worth jiggling around with the wiring harness at the same time, in case there is a broken wire in the harness shorting out.
If you've still got the old fuse, examine it closely. A normally blown fuse will show the wire inside melted. If teh wire is still intact or if it has just broken with no sign of melting, it could just be an old fuse, or vibration that's caused the prioblem.
In the meantime, make sure you carry a couple of spare fuses in the toolbox in case it blows far from home.
Graham.