Nick,
what you need is a clamp-on ampere meter capable of measuring <u>dc</u> [not ac] current down into the few milliamps range. Then attach the jaws around one + cable at a time, read the amperage flowing through that wire, jot down the number on a piece of paper - move on to the next wire/cirquit and repeat. After a while you will have checked them all out and have a neat listing on the paper telling you what is the problem...
[If it <u>is</u> a problem - modern electronics tend to use a lot of juice already in off, or sleep, mode. Compare this to yesteryear when the only 'drain' was the operation of the clock... Now radios, alarms systems and all the other add-ons we've fitted into the cars adds tens, or hundreds, of milliamperes to the steady drain of the battery when the car is parked. And with a 'weak' battery there aren't that many hundreds of hours before it goes flat if the drain is say 150 milliamperes...]
Using a clamp meter saves the work of breaking up each cirquit for checking with an ordinary in-line ampere meter and then refitting it again afterwards - doing it this way also tends to kill radio memories etc since they usually don't like to be disconnected...
[Not a solution to your immediate problem, but something to think of for the future - for most of us: It would simplify things if the fuseblock is changed out for a modern equvivalent using blade fuses - the originals are getting way old and brittle, sometimes from fitting improper size fuses: they are designed for the slightly shorter British 1/4-inch glass fuses!
For modern blade fuses there are neat testing probes on the market that substitute as the fuse for testing purposes, like this one:
http://tinyurl.com/2lbaen Makes me [at least] quite interested in a rewiring of the Wedge fuse central. Not only a better fuse design but also easier to trouble shoot...]
