An LOC is a Line Output Converter, which basically 'converts' the speaker-level signal to a line-level signal so you can use it as an input to your RCA inputs on the amp. They are available at any car audio shop or many online places. Check our
www.davidnavone.com for example (under 'adapters'), but there are many others.
You can tap into the speaker signal anywhere you can access the wires and identify them. Depending on the location you can splice your way in at the speaker, at the factory amp, or anywhere on the wire. In either case, you need some way to identify which wires connect to what. Just take any regular speaker output and connect to that, and if you're adding bass just get an amp with a built in low-pass filter.
Hope that helps, let me know if I can clarify.
T.