If you're installing a sub and you want to keep the stock head unit and HK amp you will need a multi-channel line output converter with summing. You can install the sub by just tapping into the speakers or out of the head unit but the sound is going to shit. That's how I originally had it before I installed an LC6i https://www.crutchfield.com/S-hMaGG5piG0Z/p_161LC6IB/AudioControl-LC6i-Black.html
Now my system sounds amazing.
Check out this thread @caheiss explains it well
https://legacygt.com/forums/showthread.php/starlink-head-unit-removal-schematics-237457p7.html?t=237457&page=7&highlight=lc6i