I can not recommend you do that since that battery is going to drain pretty quickly and leave you without trailer lighting and you'll have no way of knowing while driving. I know it seems like a hassle but running the power wire up to the vehicle battery is the only safe way we can recommend you install the part # 119250KIT in your 2017 Audi A3.
I attached a help article that has tips on this process for you to check out as well. view full answer...
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