Went through this on my '03 ST....
Check the female pigtail from your truck that would normally plug onto the 9008 lamp bulb. On my truck, one of the three pins/terminals was pushed back and making poor/intermittent contact.
When I was running normal 9007 lamps in my ST (before I figured out the problem), the driver side would occasionally be dim. I could wiggle the wires a little and it would start working fine again for a while. When I changed to HID's, that side would occasionally not work, and until I swapped the passenger side ballast and lamp over I thought it was a bad ballast. Upon closer inspection I found it was one of the terminals in the pigtail was pushed back. If I removed the rubber plug from the back and pushed it forward it would simply get pushed back away as soon as I plugged anything into it. So what I had to do was plug the ballast into it and then push the loose terminal forward so it slid firmly onto the male end coming from the ballast, then slip the rubber plug back into place. Been fine ever since.
