[MrSnarkyPants]

Bought my 2012 a few months ago and had the driver's side headlamp go out.

Pull into an Auto Zone, buy the factory lamp, and surprise! The former owner put in HID bulbs.

I've never dealt with these before. Do I replace just the bulb or the ballast and bulb?

Do I need to replace them as a pair?

How difficult would switching to LED be?

 

[telveer]

It could be either bad bulb (swap the other bulb and see if it works) or a bad ballast (other bulb does not work either). Bad bulbs can be replaced. I would recommend changing both, though. If it is a bad ballast, see if the item is still under warranty (contact old owner, if possible). If warranty is not available, just replace with a new kit, which can cost anywhere from $50 (generic) to $150 (morimoto kit from the retrofit source). You could also go back to HIR (OEM) bulbs, or try one of those new LEDs.

 

[Dutch]

Do I replace just the bulb or the ballast and bulb?

previous poster gave good advice, test the bulb with a known good and if it lights it was the bulb, if not then the ballast is bad (too).

Do I need to replace them as a pair?

Generally a good idea to keep the brightness and color temperature consistent. Few things look dumber than mismatched HID lights plus the visibility impact.

How difficult would switching to LED be?

Easy peasy. Most of the modules out there now are straight plug-n-play. The hardest parts of the upgrade I did to OHM-RIDE were rotating the emitters to get a good beam pattern and getting the heat sink flaps unfolded and evenly spread inside the headlight housing.

SOMMS did a more recent upgrade with a newer set of LED modules (see this writeup) In his case it looks like the re-designed modules would be even easier to install and adjust.