Help Please

Most units I have seen have a subwoofer output designed to feed a sub woofer amp etc but as I'm not really into car audio some units may have an inbuilt amp.

Powered sub woofers can also be connected to existing rear speakers for example but you won't be able to control the volume from the head unit doing that way.

Other issue is sub woofer either powered or with a seperate amp are usually pretty powerful so a good power source is required to feed them.

There are lots of variables in your question so you really need to seek professional advice from a car audio retailer as your current setup will dictate what is required

Generally you will need an amplifier as head units do not have enough power to drive a sub but you can buy subs with amps built in

Talk to a local installer, they will give you more relevant and specific advice than you are going to get online