Help-bad ballast??

[PeteG]

So, I've been having chronic problems w/ my driver's side low-beam (xenon) not turning on (often when cold and/or very wet out). Now, the low beam won't go on at all. I had a spare matching xenon bulb and just installed it. Low beam will still not go on-don't think it's the bulb now.

I'm thinking it could be the ballast-any ideas or insights? If so, is it ok to purchase just one ballast or do I need to do in pairs for some reason. If so, does anyone know what the OEM ballast is (is it Phillips)? Should I ebay it or is there another suggestion?

I have an E53 2001 x5 3.0i, original owner.

As always, thanks in advance!