I've searched for other threads and none seem to take it further than replacing the bulbs. I own a 1999 996 coupe. Last week I noticed the driver's side low beam was dim. However, the highbeam, running light, fog light and turn signal all perfect on that side. Passenger side low beam and all other lights good too. Changed the bulbs on both sides and the driver's side low beam continues to be dim. I checked the wiring in the headlamp unit and found that the plastic surrounding the multiple low beam wires was frayed with exposed wires. Not surprising considering the car is 15 years old. I separated the wires and covered the exposed areas with electrical tape. I thought I had a victory. Low beam remains dim.

 

Look for a poor ground connection, this can cause a dim light

 

Relay.