I can't remember off the top of my head, but I'm pretty sure I have it listed in the write-up I linked to earlier in this thread. I know that the lamps will only fire if the polarity going to the ballasts is right, so if they both light up you got the wires right.

Interesting point about the welding since HID lamps and arc welders generally operate on the same principle: Jump a spark across a gap between two electrodes. That tends to generate strong RF/magnetic fields, which in turn would induce noise in any other conductors in the general vicinity. If I remember correctly, that is exactly what Marconi's (or is that Tesla's) first radio set did!

I wonder if the lamps were sourced from a different manufacturer? Could it be something about their construction?