Here is a link from the Mercury forums:
http://www.automotiveforums.com/vbul...d.php?t=536985
The issue of cloudy, dull, yellowish headlight lenses is addressed here. There is a way to polish the yellowish residue off of the headlamp lenses, which should work in the same manner for the Park Avenue. Also, you can remove the lenses and the sockets from them to clean the inside, I've done this before. If you do this, make sure to have your lights readjusted before night driving again!