1990 300 SEL Changing Headlight bulb
 

This might be silly, but how does the old bulb come out? Does it screw out or just pull out straight?. It appears that a white plastic thing is holding it in place and I don't want to break it.

The plastic retainer needs to be turned in order to release the bulb. It can be stubborn, and I forgot if it's turned clockwise or counterclockwise. :confused:

Matt, turn the white plastic retainer ring CCW. You may need a long screw driver or piece of wood to loosen it as they can be difficult to turn. Be careful not to use so much force that you break off one of the wings.

I had this same problem with my headlights, the plastic melted around the mounting area. The bulb heats up and melts it just enough to make it stubborn. If your other light is not burned out yet be sure to replace it as well, the newer bulbs have better plastic mounts and if you replace it before it burns out, you can turn it on to heat it up first making removal very easy. I spent a hour trying to get mine loose, It drove me nuts. like the others said be careful to not break the mount, it wasn't until I got it out I realized it was melted, if I had known that I might of taken a hair dryer to it to warm it up or something. Good luck!