If only the low-beam is out, you will need to replace the low-beam bulb. It is probably not necessary to replace the entire headlight assembly.
how to replace 2007 Mercedes GL450 Headlight Bulb Low Beam
A sealed beam headlight is just that. The unit is sealed and you replace the entire light. A composite is made of pieces that's why the name. To replace a composite you put a new blub in the back.
Carry it to dealer who will replace on low beam headlight for $15 +/- dollars in about 5 minutes.
headlight switch is probaly your trouble
The headlight has two filaments. One for the high beam and one for the low beam. The most likely reason the high beam is out is because the high beam filament is broken. Replace the bulb and it should fix it.
replace the high beam bulb
The Low Beam AND High Beam are in the same bulb. The procedure to change it will be in the owners manual.
Depending on the model year, you probably have one low beam bulb and one high beam bulb in each headlight.... unlike 90-94 323s that had one 2-filament bulb in each headlight.
replace passenger side low beam headlight
My owner manual list both the high beam and the low beam as H1, 55 watt bulbs. Your local parts store or the Mazda dealer can confirm this info for you.
replace the low beam headlight bulb