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In order to diagnose this problem you will need a volt meter and some basic electrical\mechanical skills. Do you have a volt meter so we can start testing to find the problem?
OK I would like to note that I will be available for an hour now and then tomorrow morning until 10:30am central time then back here at 9:30PM central time(I go to work). I don't want to leave you waiting for an answer, I will get back to you as soon as I can.
Now let's get to the problem, I would like you to start by locating the brake switch. Connect your black meter lead to a ground and check with the red. First check the green wire with a black tracer, It should have 12v, then while checking it turn your headlights on and see what you read. Then connect your red meter lead to a green wire. This wire should read 12v when the brake pedal is pressed, Turn your headlights on and see if you have 12v or it goes away.
Let me know what you find, I want you to check this to figure out if our problem is with power supply or it's a ground problem.
Let me know what you find and I will get back to you as soon as I can, Hopefully it will always be fast.
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Please let me know if you have any other questions and thank you for choosing just answer.