What caused my headlamps to melt and now have nicotine cover leaky surface?

What is your question?

The lamp covers to my headlights have continued to melt, causeing an ugly eyesore to the covers. Appears they have melted. Could the bulb be a higher voltage then it should be and also, are there any specific type of bulbs that should be installed? I wonder if the dealership could have replaced the bulbs with cheaper version? The older the car gets, the MORE MELTING OF LAMP COVERS! I UNDERSTAND THE COVERS ARE OF PLASTIC AND WILL CONTINE TO BE COVERED WITH WHAT APPEARS TO BE A DARK NICOTINE COLOR..ANYONE HAVE THIS ISSUE? PLEASE HELP AND THANK YOU SO MUCH! IT STARTED WITH ONLY ONE HEADLAMP AND NOW THE OTHER ONE IS STARTING TO DO SAME THING, AFTER HAVING A LAMP BULB REPLACED. WILL THIS EROSION BE ABLE TO BE REMOVED OR WILL THE ENTIRE LAMP HAVE TO BE REPLACED?