Are all of your signals stock, or have you replaced any with LEDs?
Same question with dash indicator lights (the light that illuminates the spot on the dash, esp for the turn signal).
Technically speaking, our turn signals are all one circuit, with the dashboard light in the middle. Power is introduced at either one end or the other (depending on which way you flick the switch), and the circuit relies on cumulative resistance to prevent the lights on the other side from flashing.
Assuming you answered 'no' to both questions above, my initial (and easiest to check) guess would be that one or both of your rear turn signal bulbs is blown out. You can check by removing the screw on the back of the reflector housing, removing the lens, then gently twisting the bulb, followed by pulling on it. If the bulb has any parts moving about (more than the tiniest amount when gently shaken), it's almost certainly blown. Your local auto parts store (or likely even super-store with an auto section) will have a replacement, probably <$5.
Second guess would be that your turn signal dash indicator light (the one that's behind the green dot on your dashboard) is blown out. That's a little harder to get to, but essentially it's in a rubber boot. Here's a pic of the back of the dash, it's in the boot indicated with the red arrow.
DSC07224a.jpg
If you've got small hands/arms (or a helper with such) you don't have to disassemble anything to get to this bulb, but it's a little tricky, and make sure to NOT pull on the wires. (Also don't worry if your wires don't look exactly like mine, I've done a little modding in there). Same deal for checking the bulb as above; if the filament moves more than a little (or isn't attached at all), it's blown and you can get a replacement cheaply from your local auto-mart.
If you answered "yes" to either question above, you've likely got a resistance issue that will need the diode mod or an electronic flasher relay to fix, if not both. Just search on either of those term, you should find plenty of results