The retaining wall site modifier creates pads all around the wall, and you can control the elevation of these pads so that the soil is retained just as you want it. Under the wall, the modifier creates a flat pad, which is slightly offset from the edge of the wall, so that you don't get modifiers touching at different elevations and consequently get some of them ignored. The pad for the parking area should be inside the retaining wall, but not touching the modifier. If it DOES touch at the same elevation,you will receive an error message, but in fact, it won't make any difference to the result, because it is touching at the same elevation. Where you have two points touching (in plan), but at different elevations, one set of points will be ignored.
Hope that helps