Congratulations, you've been selected to receive one of the toughest troubles to shoot!
Basically, you have to find and isolate the wire or device where the short is.
You need a wiring schematic, a VOM (volt-ohmmeter) and some ingenuity. At the fuse block, remove the fuse and measure the resistance to ground from LOAD side of the fuse. A short will give you a very low reading to ground. CAUTION: Be sure which is battery side and which is load side. Measuring the battery side can "smoke" the meter!
How low is low? Normal resistance for 10A is 1.4 ohms or so, a 20A is 0.7 ohms, etc. The formula is Resistance = Volts/Amps where volts can be up to 14 in a normal car. These numbers will vary a little, as a 20A fuse doesn't necessarily blow at 21A!
Good, you say you found a low resistance at the fuse block? OK, check the schematic, find the first connector, open it and check each wire that leaves it. Remember, you're still measuring resistance to ground. Again, one wire should read really low, the rest should be normal. That's the path with the short.
Check the schematic again to find where the next wire goes, find the connector, etc. Keep tracing until there are no more paths for the short. That path, or device, has the problem. Let's say the wire to the door lamp tests "short." The last check is to test from the lamp socket end. Still "shorted?" There's a pinched wire between the socket and the connector. No "short" if you remove the socket? Maybe it's a bad socket.
I hope this makes sense. It's easy to trace on a schematic, and its more exciting on the car, where most of the fun is to find the connector!
Good luck on the detective work!