Now's an excellent time to learn troubleshooting procedures.
You've gotta think like Napoleon - Divide and Conquer! (or was that Atilla the Hun?)
But for you to do that, you've gotta know something about your intended victim.

In this case - learn how to read a wiring diagram. They're not exactly the easiest to figure out, but after a while you'll get the hang of it. They can actually be kind of fun - and the thing of it is, the only other way to figure out what's going on with your wiring is to trace down every wire and figure out where it goes, and whether it branched somewhere when you weren't looking. So to work on any electrical stuff, it's almost mandatory reading the wiring diagram. Without it, I'm lost. Most of the time that's the main reason I get a manual - just for the wiring diagram!
If you've located the fuses in the diagram above - they're HUGE, so how could anyone miss them, right??

You'll also notice that the one for the fuses has two wires leaving the fuse.
There's a good chance, one of them is the trouble maker! Disconnect one of them and see if the problem persists.
However, at this point, a little understanding of meters and electricity is necessary. You could use a trouble light, but an understanding of electricity is still needed.
For electricity to flow, a complete circuit is necessary from the positive terminal of the battery back to the negative. If there's a break anywhere in the circuit electricity won't flow.
The easiest one to explain is a trouble light, so that's what I'll do. A trouble light is a light bulb attached to a battery, with leads from positive to negative. If you touch the two leads together, electricity will flow and you'll know it because the bulb will light.
In this case, because the fuse is blowing, you know that there's a place in the circuit that has a short to ground. Because of that short, too much electricity is flowing through that fuse, it heats up, melts the internal wire which then melts and suddenly no more circuit to ground!
So what you want to do is hook the black lead of the battery to the negative battery terminal or somewhere that has a good ground like a cylinder fin or a bare spot on the frame (unlikely). Then you'll touch the other lead (usually something that looks like an ice pick) to one of the leads you removed from the fuse connection. If the bulb lights up, that tells you there's a path to ground when there shouldn't be - afterall, the power is off so nothing should be happening, right?
If the bulb doesn't light, then you know that circuit is ok. Check the other one.
If neither test positive, then it's most likely one of the turnsignal leads heading to the bulbs. There again, you can divide and conquer to eliminate the good one and weed out the bad. Disconnect the wiring harness to the rear lights and see if there's a problem. Does the fuse blow? If so, then it's not in the rears.
You get the idea?