Take a dc voltmeter to the back side of your factory parking light switch and figure out which terminal is energized when you turn on and off the parking lights. Do this by grounding one lead and connecting the other to each wire terminal, one at a time, while turning the parking lights on and off. If you have 12 volts the following will work. If not, quit reading here and do not attempt the following because your switch might be connected to the cars computer and you do not want to fry it. Buy yourself a 12VDC relay with normally open contacts from your local electrical supply store. Wire one side of the coil to ground and the other to the parking lights wire you found earlier. Now, I'm bottomuming you have already installed and wired your offroad lights, cut the wire on one side of your new offroad lights on/off switch. Connect one end of the cut wire to one side of the normally open contact on the relay, and connect the other end to the other side of the same contact. Now if you did this correctly (and didn't let out any of the magic smoke), the offroad lights cannot be turned on when the parking lights are off. If this doesn't make since, any electrician worth his salt can help.
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