My wife wants brighter headlights in her ‘06 ML350.
what can be done by changing bulbs only that won’t mess up the electrical?

 

The sun causes the plastic headlamps to darken. Pick your poison.

https://www.youtube.com/results?sear...dlamp+cleaning

 

Does your ML350 have halogen or HID headlights?

If you want maximum output, a retrofit is your best option, though it takes a fair bit of work. See this: https://mbworld.org/forums/r-class-w...-retrofit.html

If your headlight lens are yellowed, the restoration procedure will bring back some brightness. Keep in mind that you will need to do this about once a year once you start.

If you have halogen headlights, there are brighter bulbs available with the tradeoff of shortened lifespan. Halogen bulbs also get dimmer as they approach end of life.