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Hello I will help you with your question,
If you want enough power to run a heater you will need to install a dedicated circuit straight off the battery. A fuse at the battery and 12 gauge wiring properly protected to the rear will give you 20 reliable amps to use. I don't know ifthat would run a heaer but it will drain a battery fairly fast.
Please let me know what you are thinking of so I can help