Hello I will help you with your question,

Try cycling the turn lights and hazard quickly, the brake lights feed through the turn switch to get to the rear lighting but the high mount light does not.

You will need a 12 volt testlight to check that you have power on the light blue wire running into the switch when the brake switch is on. If you do have power then replace the switch

If you would like any additional information or have more questions please don't hesitate to ask!

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