You are making the testing of the turn signal switch much to complicated.
While in the truck, test the function of the wire at the connection under the dash while everything is still plugged in and powered up. No disassembly required. Leave everything hooked up as it would be if working.
You will either have power at the front left turn connection when the left turn signal is on, or you won't. If you do your test light will flash.
If it does not, turn on the hazards, if the test light flashes then, the problem is in the switch.
