Just the left. It triggers the "check lowbeams" warning on the dash when I shut the car off, so it sends the fault. I'm betting you're correct on the ballast. I'll just order a new pair and replace both.

__________________
1978 E21 320i 1980-99
1985 E28 533i 1999-2001
2006 E53 X5 4.4i Build Date 6/06 2012-