For the 2 power wires for the new headunit (12V+ Constant should be yellow on your aftermarket radio harness, 12V+ Switched should be red on your aftermarket radio harness). Get a test light out or a multimeter and find a constant 12V+ (Terminal should have 12V+ no matter what position the ignition switch is in.) on the junction block to by the drivers side kick panel, run the yellow wire from your new headunit to a terminal that is always hot. Next find a terminal on the junction block that is ONLY hot when the key is in the "ACC" or "ON" postion. It wouldn't be a bad idea to fuse the yellow wire, as the junction block is not fused.
The OEM speaker wire is junk IMO, so just run new speaker wire to your front speakers from your new headunit, and then locate the rear channel wires in the OEM radio plug. There is 2 or 3 plugs that plug into the radio, but only 1 has all of those color wires on for the speakers. It should not be hard to locate at all, hope that helps!