I didn't... The Sony is screwed in through the holes already in the stock bracket, using the screws Sony provided - that part went right together. The empeg was lined up by eye and then tack welded into place (by tack welding I mean it was hit in four corners for just a second each - it's secure but would be fairly easy to grind off to switch cars). The on the bezel of the car, I carefully hit it with a Dremel and later on with varying grades of sandpaper and a sanding block to keep it flat.

My only regret is that I cut the sides of the bezel first where the empeg would slide back into to keep it flush. It was after I cut those that I realized I needed to cut some from the top of the opening as well. Once I did that, it turned out that the side cuts that once were perfect now extended too far down. Apparently I'm the only that notices, and by now I have pretty much forgotten about it.