Thanks for the links. I've actually been searching on diyma as well. I tried reseating the connectors several times, but that didn't help. Then I tried to tenporarily connect a second ground wire from the back of the HU to the same ground post the amp's connected to. That also didn't help.
I didn't cut the factory ground wire at the back of the HU though. Am I supposed to cut it? Before I do any cutting, I just wanted to be sure.