Hi and thank you for your question.
Since you have narrowed it down to your left turn signal circuit, we need to divide and conquer the left side. My first suggestion is to remove the front blub and turn the left turn signal on, and see if it blows a fuse. As an alternative to going through a bunch of fuses, you can connect an ammeter in place of a fuse and when you power up the left turn signal, watch the ammeter and see if it is going over the rated amperage of the fuse. If it is, immediately turn it off.
Put that bulb back in and remove the rear bulb. Test for amp draw again. If you get excessive amp draw with either the front or rear bulbs out, the final test would be leave both bulbs out and test again.
If it still draws excessive amps, you have a pinched hot wire going to one of the two turn signals. To narrow down which one it is, unplug the rear TS and re-test for amp draw. If you are still getting excessive amp draw, unplug the front TS and re-test.
What we are looking for now is what affects the problem. If we unplug a component and the amp draw goes away, we have narrowed down exactly where the problem is. If you still have excessive amp draw with the F&R TS unplugged, the problem is in the wires between the Handle bar switch and the connections in the harness for the turn signals.
I know you said you inspected the bulbs carefully, which is a good thing. I have seen bulbs that had internal shorts create these symptoms. I have also seen bulbs installed in correctly and cause a short as well. If the bulb has a running light and a turn signal element in it, it has to be installed correctly.
Give that a shot and let me know if you need more help.