I have a grill that has running lights on it and I saw the headlight wires have a gray blue wire that’s for running lights but when I splice the wire they don’t work why?

 

Welcome to the forum

Use a Volt meter(or test light) and test the wire for 12volts with Park lights on, if no 12v then that is not what its for, could be for driving lights or fog lights or ??

If you have 12v then you may be using LEDs bulbs in the new park lights, and they have a polarity, a + and - wire, so need to be hooked up correctly to work