On a related note, our 164s here have the Elma headlights and H4 bulbs. When I bought my last 164 (from Singapore), it had somehow been fitted with a left-hand-drive headlight on the left side. The bulb was put in wonkily so as to give a pool of light that didn't angle towards oncoming traffic. This passed inspections but was awful to drive by - absolutely no illumination of road signs.
Anyway, both headlights were 'cloudy' inside, so I decided to wash them. A solution of clothes-washing powder and hot water got the inside of the glass sparkling clean BUT the water emptied out in a silvery stream. That's right - it took the silver off the reflector!
Unfortunately I only noticed this once the other side was in the wash as well. The silver was missing at the base of the reflector back surface, in both headlights.
I wasn't so worried about the LHD headlight, as I figured that needed replacing anyway. Then I discovered that they're $350 (US$200?) second-hand.
I located a company that could re-plate the plastic reflectors. I tried to remove the right (i.e. a correct RHD) lens, and cracked it.
At this point I gave up on the whole job and cried into my Chianti (figuratively at least). The headlights of my 164 were always hopeless.
My current 164 has two correct (RHD) H4 headlights, with no cracks. I haven't entertained the thought of washing them yet. Do you think there is a 'safe' way to wash them?