This, the '06s use H11 low beam. It's just easier to upgrade to HIDs since you have to buy H11 bulbs and connectors if you go halogen.
All (or most) HID ballasts on the other hand use 9006 connectors flipped upside down, so you buy an H11 HID kit and they're just plug & play. i.e. connector plugs into '03 9006 connector, bulb fits in '06 H11 housing.
edit - you don't *have* to buy the wiring harness, all it does is give you a second set of turn signals and make the parking light connector a little better. The '03 parking light will work in the '06 housing, it just fits a little funny. Once you turn it 90 degrees to lock it in it snugs up firm though.