Test the voltage at the socket I bet it isn't what it should be. I think it should be 6 volts maybe someone can chime in. Many grand marquis & crown vic owners installed a relay inline to fix the problem. Not getting enough voltage to illuminate the bulbs to their brightest is acommon problem other members just went to HIDs.
2004 marauder black purchased new. Sold 9/16 at 297,000 miles. Still ran strong didn't burn a drop of oil.
2003 marauder black 28,000 miles all stock. Garage Queen.
2004 marauder red daily driver.