Not familiar with the stock radio but generally speaking, most don't have line-level outputs and the only way to add a subwoofer is to use the speaker-level output from the rear speakers.

If your HU does not have line-level outputs whatsoever, look for a subwoofer setup that can take speaker-level or high-level signals.... whether it's an all-in-one active sub or a passive sub with an outboard amp. You will then reroute the rear speaker signals to the sub amp. With this setup, you can control the subwoofer level using the fader control on your HU, among other means.,

Optionally, you can get an audio transformer to convert the speaker-level signal to line-level.... it all depends on what you currently have to work with.