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<blockquote data-quote="Stel" data-source="post: 4175482" data-attributes="member: 574698">I would use a &quot;single pole double throw&quot; type. Here&#039;s a diagram (the one labeled SPDT):<a href="http://en.wikipedia.org/wiki/Relay#Pole_.26_Throw" target="_blank">http://en.wikipedia.org/wiki/Relay#Pole_.26_Throw</a>NOTE: for the A terminal I mean the one that is normally closed (completing the circuit) when no power is supplied to the coil. You can use a multimeter to be sure which is which.Connect the existing +12v feed in the door to the A terminal and connect the light +12v input to the C terminal (this is the &quot;common&quot; terminal). Connect the fog switch (which is also +12v) to one of the coil terminals and to the B terminal. Connect the other coil terminal to ground. This way with the fog switch off and no power to the relay coil, the circuit behaves exactly as before, with the fog switch on the coil turns on and opens the old circuit and closes the new circuit which turns on the light.</blockquote>

[QUOTE="Stel, post: 4175482, member: 574698"] I would use a "single pole double throw" type. Here's a diagram (the one labeled SPDT): [URL="http://en.wikipedia.org/wiki/Relay#Pole_.26_Throw"]http://en.wikipedia.org/wiki/Relay#Pole_.26_Throw[/URL] NOTE: for the A terminal I mean the one that is normally closed (completing the circuit) when no power is supplied to the coil. You can use a multimeter to be sure which is which. Connect the existing +12v feed in the door to the A terminal and connect the light +12v input to the C terminal (this is the "common" terminal). Connect the fog switch (which is also +12v) to one of the coil terminals and to the B terminal. Connect the other coil terminal to ground. This way with the fog switch off and no power to the relay coil, the circuit behaves exactly as before, with the fog switch on the coil turns on and opens the old circuit and closes the new circuit which turns on the light. [/QUOTE]

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