Turn signal problem

Hi all ,
I have an odd turn signal problem My left rear signal does not work, the front one does, I have replaced the bulb and also the light holder (small circuit board), and still no go. I do get an indication that the light is out by the fast past clicking on the turn indicator on my dash. 96 GMC sierra 1500.
So my question is where does the turn indicator get its information? so i have a bad wire somewhere? Any hints would be appreciated.
regards Suresh

Your directional switch may be bad
Suresh Rasaretnam wrote:
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For example, if the flasher circuitry's input sees less current or more load than normal it, in turns, causes the timer circuitry to blink faster. If I remember correctly, two or more bulbs cause a normal blinks. One bulb causes rapid blinks. You can bench-test your flasher timer with this rule.

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Thanks for catching my error. Yes, "less current = less load" similar to your statement.
On an IC controlled flasher, lamp outage sensing is designed to sense whether there is adequate current being demanded by the combined light bulb loads, and, if not, the flasher will increase the flash frequency by a typical ratio of 2:1. The method of sensing the above described "low load" condition involves a current sensing circuit which monitors the voltage drop across a very low ohmic value resistor.
There is also an internal reference voltage which is compared against the voltage drop across the sense resistor, and the result of that comparison determines if the oscillator will run at "normal" rate or at "double rate" for bulb outage detection. Below are a few flashers:
Thermal (Thermo-Couple) shown here, as described by Dick:
http://auto.howstuffworks.com/turn-signal2.htm
Electro-Mechanical Electronic (IC controlled) described above.