Hey Turk,
For this job, i created a duplicate layer and hid the original (just so i don't ruin the original in case i need to start over).
I used the Clone stamp tool as opposed to the healing brush, and sampled cleaner areas of the car that had similar color to the area that i was using it on.
I adjusted the Flow down until i got a level that wasn't sampling too much of the other parts of the car, but enough for the darker clean black to show up and just dotted the dirty areas slowly, some more than others.
Once i was done sampling, some of the lines between the panels (ie: skirt to door, front quarter to door) were very difficult to make out, on a new layer i used the brush tool, set to 100% flow, using pure black color and a very fine point(maybe 3px or so but with feathered edges) and drew in the lines for the creases that seemed to have been sampled over or were just hard to see, i then duplicated the layer, changed the lightness of the layer to +100 in the hue and saturation menu to make it white, and moved it one pixel right/left/up or down depending on the direction of the light.
Then just used the eraser on a large size (something like 50px with a feathered edge and flow down around 16% to rub away at the white and black lines i had drawn until they blended in enough to look like the actual lines of the car should.
Finally i merged all of those layers once i was happy with it and duplicated the resulting layer, set the layer style to multiply to bring out the dark tones, decreased the opacity of that layer until i got a nice smooth black that wasn't too dark (just looking at the paint and not the rest of the image).
Then used the eraser again on quite a large size, around 70px or so with a feathered edge, and on 100% flow to remove the majority of the out side of the image leaving just the car (keep in mind the original merged layer is still back there).
And to finish, i decreased the eraser size to a few different sizes using around 30% flow, depending on the area i wanted to erase, and started rubbing the paint sections back where there were reflections, as well as through the window and on the driver, the rims, headlights, tail lights, just anywhere that should be shiny.
And that is is for the most part, i probably made some alterations here and there, i think i added a gradient down the bottom to create the shadow you would see going under the car, but that is the gist of it.
-Steve