I have an 03 chevy truck, when I apply the brakes the 3rd light on the cab comes on but the other two will not. I have replaced the bulbs,fuses, brakelight switch, the multifunction switch & see no dammage to the harness. What else is there to do?
check the wires themselves that run from the harness to the back taillights check voltage and resistance and look for an open circut or broken wire
your 4 way flasher may be bad ...
then theres always the obvious , do you have a bad ground ? do the 4 ways flash ? you got right and left turn signal ? if yes to both you missed a switch somewhere .
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