Replaced HDD. No Display and single beep.

The HDD in this iMac was failing so I replaced it with a known good, apple certified HDD. After reconnected the HDD and reconnecting the LCD, I powered on the iMac. I can hear the fans power on, but there is no display on the LCD. After a few second I get a single beep every 5 seconds. Looking up beep codes says that this means RAM is not present. I'm not sure why it's doing this since I didn't go anywhere near the RAM.

I've double checked the LCD connector and the only issue I have is with the display power connector. If I unplug it and power on, the LCD remains blank but the DVD drive whirs and I get the startup "bong". I'm wondering if I'm not plugging in the display power cable correctly. The socket it goes in to has 8 pins and the cable has 6 pins. Should it be plugged in dead center or to the left-most pins or the right-most pins?