I know this is a common problem in older vehicles with the dimmer switch integrated in the headlight switch, but the metal coil for the dimmer part can break and get corroded making the headlights fail. I recently had this issue with my 90 SC and it actually shorted out the headlights while I was driving at night. I kept noticing this burning smell and it finally started smoking. I pulled over and noticed it was coming from the headlight switch since the dashboard got really hot. Thankfully the local Ford dealership still had the headlight switch available and it only took me 5 minutes to install. So if you notice a plastic burning smell only when you have your lights on, this could be the issue. It's an easy part to replace.