This is difficult to answer, but I'd assume it depends on the vehicle. Have you considered doing it yourself? For some reason, people are afraid of installing components and LOC's themselves. It's as simple as tapping into the stock harness, and running the wires (or tap back in to them from another location) back to the needed speakers.
Components genrally have an "Input +/-," and then a M and T Output. The input is the signal wire coming from the stock output. Then you run wires from the Midbass (M) and Tweeter (T) to the actual driver.
Simple, one input, and two outputs.
The location of the tweeter is strictly user preference, and I prefer to have mine in the A pillar angled to bounce the signal off of the windshield.