Can I install a powered subwoofer without a line out converter?

Getting my sub installed and I’ve been told different things by different shops about whether or not I need an LoC. I was told that I wouldn’t need one because they could tap into the cars speakers but I don’t have any rear speakers so I was curious how that would work. My car is a 24 Mazda 3 without the Bose system and I’ll be keeping the stock radio. Any help or thoughts would be greatly appreciated. Thanks.