Your logic is correct. The problem is I cannot see anything on the diagram to suggest a cause for this. The dash indicators and the exterior lights are switched on by different parts of the signal switch, but all of them get the same pulsing 12 volts from the signal flasher. That comes in to the switch on the purple wire. Use a test light to test for the pulsing 12 volts on the light blue / white wire when switched to the left turn position. That is the line going to the indicator and will simply verify the test light has good connections.
Next, check on the yellow wire. If there is no pulsing 12 volts there, or on the dark green in the right-turn position, check that the hazard switch is not stuck between positions.
Sunday, April 16th, 2017 AT 2:00 AM