It's possible that a headlight relay could make your lights brighter if it handles the electrical current with a lower voltage drop than the normal circuit (through the headlight switch) does. Meaning the relay has more current-handling capacity and less resistance.
Before you go to all the trouble of buying and intalling a headlight relay, you can run a simple test to see if it will help. Carefully hook up a temporary jumper wire directly from the battery to the headlight terminal. You might even be able to do this with the regular connector in place - I don't know for sure since I haven't tried it on a MB. But the idea is to directly provide a good circuit from the battery to the headlight and see how much brighter the light gets. It's similar to just using a voltmeter at the battery to see how much voltage is being delivered to the headlight.
Also, the other part of the electrical circuit is the ground. I've seen people spend countless hours and money trying to improve the voltage being delivered to the headlights, when they had a bad ground. You can test the ground too by just jumpering a wire from the headlight directly to the car's frame.
Naturally, to do all this, you have to know which pins are + voltage and which are - voltage. You don't want to create a short!
Just about any 12V relay with the proper current handling capacity will work as a headlight relay. I think you would want one that is sealed in a plastic case somehow. Two 100-watt bulbs will draw something on the order of 17 amps, so you need at least a 20-amp capacity relay. You use the existing electrical circuit that is switched by the headlight switch to throw the relay. The switched relay terminals have one side connected directly to the battery and on side to the headlights. (You may only need one relay for the high-beam circuit - or you may need a second relay for the low-beam circuit also if its really dim.)
A previous owner burned out the headlight wiring in my W124, possibly with high-wattage headlights. The dealer charged him a pretty penny to fix it. If I ever go with new high-wattage headlights, I'll be using a relay to protect the stock circuit.