http://www.bmwe34.net/e34main/upgrade/more_light.htm Try this. I did and it helped. See the bottom of the page too. Cleaning will help.
This from the E-36 site. They say to remove the light but I'm not sure you couldn't do it in place with a coat hanger if you removed the bulb.
You'll need clean titrated water to dilute *glass-cleaning* liquid - they come in many brands. Next, you'll need a length of *steel* wire about 1/8th" thk. Formed it to a contour of the reflector on one end and the lens on the other end - long enough to grab it in the centre. Make sure you file/polish the ends round and smooth.
Now, get strips of *fluff-free* clean/new cloth - preferably cotton and tightly wrap the whole length of the *formed steel wire* such that they are thicker along the 2-3 inch length on both ends. Try to insert the ends thro the bulb socket - easier if the lights uses H4, tight fit if H1.
The process of cleaning is such that you wet the tool made, just sufficiently moist - not dripping, and insert - clean! It will take some effort until you get the best result. Change the strip of *fluff-free* cotton strips regularly during the cleaning process.
The reflector score easily - so do be gently!
I had been doing such things all along. <grin> Of course, if the lens too *blurred* due to chippings, replace with new... easier!
Cheers!
ps. If the rubber protective caps that seal the bulb over the light socket looks aged, replace them. Water can and do *seeps* inside the light unit, due to *convection* set up when light unit warms up and then cools.