help with line level

What do you mean when you say "the line level kit"?  Is this a monte-carlo specific thing, or did you just mean you have a line out converter?

To add a sub amp to this stereo you have to run the turn-on from either a switched outlet at the fusebox, or the accessory wire in the ignition harness (should be orange but verify first with a DMM).  You can't get the switched +12 volt signal from the back of the factory radio because since it is the class II wiring system it doesn't even use a switched wire.

To get the actual sound signal, you can either tap in to a pair of rear speakers or the factory subwoofer, and use a high-level to low-level converter...or you can try and grab a low-level signal from the back of the head unit itself but its level might not be compatible with your aftermarket sub amp without using an interface to adjust it.

All I know is this harness plugs into the same connector that plugs into the back of your factory radio, so if you decide to get your signal from there, just compare the harness in this picture to your harness, and find out which factory colors correspond to which functions.  The coloring of the wiring harness in the picture follows the EIA standard.

Yea your harness should have the same size/shape connector, but there will be more wires in it.

If I were you I'd keep the factory system intact and hooked up, because you'll want to get your high-level signal from either the rear speakers or, if possible, the subwoofer's speaker terminals.  You just want to make sure you're tapping in to a signal that gets all possible bass frequencies and is not crossed over or high-pass only.