There is a big piece of metal spot welded into the ends of the stock bars, which helps reduce vibration. At least it was there on the KDX's I've owned (never had an '03 though).
It's a PITA that Kawi put them in there, but it is what it is.
You will probably need to get it out to mount your mirror. If you are putting on bark busters or narrowing your bars, you would need to take it out anyways.
What has worked for me is this: Drill a tap hole. Get a small lag bolt that you can fit a socket wrench onto. Turn the lag bolt into the hole you drilled until the spot welds break loose and the metal chunk plops out of your bar. You will need some torque to break it loose (a small screw with a screwdriver is not going to do it, IMO). File any remaining bits out as necesary to get your new parts to fit.
Of course, you could take this opportunity to get new, stronger bars.