A headlight relay won't make your lights brighter. They will only be as bright as your battery has voltage.
The point to using a headlight relay is that if you really want some light you will use some form of higher wattage bulb or combinations of bulbs. Originally all the current goes through the headlight switch and the combination switch. The combination switch will have its life reduced by carrying more current than MB intended. To use a relay one would power the bulbs straight from a fused battery lead using the original headlight wire to only turn on the relay. This will bring the current going through the combination switch to less than a half amp. The original set-up places 6-10 amps with each settings. Higher wattage bulbs could take that current to 15-20amps. The switch won't like this but relays are designed to take that much current easily.
Bosch Master, ASE Master, L1
33 years MB technician