The headlight lenses on my '97 T&C seem to be cloudy (or filmy) from the
inside.
That's entirely possible.
I've cleaned the outside best I can, but it didn't much improve
things.
If the outside is indeed cloudy (it'll have a yellowish/brownish/white
appearance if so), you have to proceed very carefully. It's not plain
polycarbonate; it's got a hardcoat applied to proof it against UV (sun)
and abrasion damage. The hardcoats vary widely in their effectiveness and
durability, but if you scrub them off with agressive or repeated abrasive
treatments, the lenses will quickly get much worse. Use a good grade of
car polish -- I've had fine results with blue-label TR3 from the parts
store -- and carefully polish the lenses. Others have tried ordinary
toothpaste with success. Just don't overdo it.
Is that a sealed unit so that it's not possible to get to the
back side of the lens?
You cannot remove the lens and clean it separately, no.
Probably not. There are several reasons why your headlamps might have a
cloudy appearance from the inside. One of the most common is "gassing",
where plastic and elastomeric materials in the headlamp and bulb offgas
and deposit a film on the lens and reflector. Same process by which the
inside of your windshield grows filmy.
To degas the headlamps, you'll have to remove them from the vehicle. Take
them to the kitchen sink and clean the exterior (including the back
surfaces) of the lamps to remove excess dirt and grime before removing the
bulbs.
Remove the headlamp and park/turn bulbs. Place one or two drops of
ordinary dishwashing liquid (Joy, Dawn, Palmolive -- make sure it is the
type you can see through, not the "lotion" or "hand conditioning" type!)
in each headlamp, and then pour in hot-but-not-boiling distilled water.
Vigorously slosh the headlamps around over the sink. Repeat the procedure
once more with one drop of soap, then follow up with hot distilled water
rinses until all traces of suds are gone.
Hold the headlamp (one at a time) with the lens facing you, and swing the
lamp from over your head down to your knee level, to force water out
through the bulb holes. Once you've shaken out all the water you can, set
the lamps in a warm, dry location, bulb holes down, and allow them to dry
completely before reinstalling them. If you cannot find a naturally warm
and dry location, you can use your kitchen oven if it is electric. Simply
put the headlamps on the rack, bulb holes down. Close the oven door and
turn the oven control to 300 degrees for sixty seconds, then turn the oven
off and leave it off and closed until the headlamps are dry.
DS