I have successfully to make a plug n play wiring harness for sti non-HID projector.

However, I have a question when I install a HID version sti projector. Is the homemade harness ( uses 18 guage wires) is good enough to connect a high current HID ballast from stock plug( 9007).

Also, there was a negative wired from bulb to the ballast. Currently I didn't connect it to the ground, just leave it alone cuz the harness has negative wire already. So do I still have to connect the extra negative wire to the ground.

The harness I am using for non-HID is work on HID's, but the HID is not bright as I expected. Will the wire affect the brightness?

 

The ballast itself doesn't draw a lot. As a matter of fact it draws less than your stock bulb.

I think that having the positive and ground connected properly is important. Not sure how to know how to, but check your HID manual.

 

The HID ballasts draw less current than stock wattage bulbs, about 35W vs. 55W, when they are operating. However when the ballasts are initially turned on they draw a much higher startup current and the wiring must be able to handle this. The factory switched ground system can barely handle the stock headlight current and the HIDs should not be connected to it. The negative side of the ballasts should be connected to a direct ground, either battery negative or chassis.
As for brightness HIDs aren't like regular bulbs that can have varying brightness depending on voltage, they will be either on or off depending on whether there is sufficient power available to start the ballasts and keep them running. If the voltage drops below the minimum required to keep the ballasts on the lights will flicker and/or go out.