You'll have to run them through a relay, they will be far too high current to run from anything else.
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Take a wire from the cigarette lighter, to a switch, to the relay.
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Take a high current feed straight off the battery to the other side of the relay.
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I fitted an aftermarket seat heater kit into my MK2 and it required about 30A with both seats on high heat
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- take that from the cigarette lighter and you'll be popping fuses left right and centre!
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I'm not sure what you're planning to do about switches
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(retain the MK3 switches or fit custom ones) but you can easily wire an LED into the switch as well, so you know when you've left them on
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(or more accurately, when your passenger has left theirs on
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- it should be fairly obvious when yours is on!) which I did in my install.
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