I don't have the rear "subs and amp". How do I connect the amp and Line Out Converter? Can I connect the LOC to the harness coming off the HU?

If you're running a line converter just tap the positive from one rear speaker and the negative of the other rear speaker. The rear doors are pretty easy to take apart. There's two screw under that plastic cover under the door handle (if I recall it's hindged on the bottom so pry from the top to open it) and 2 or 3 screws on the bottom edge of the door panel.

If you want to save yourself a headache, just get an aftermarket HU with a sub-out. You can get pretty nice ones for around $200 plus roughly $30 in parts installed. Most installers are happy to pull the wires for you from the HU to the trunk for an extra $15 if you provide the wires/rca cables. If you just want to add only sub and amp then you'd need them to pull a remote wire and a pair of RCA cables for ya. Then just tap the battery in the trunk for your +/-, mount the amp to something and you're ready to bump. ;)