Recently bought a 2011 ML350. Already has towing hitch and Jack for trailer plug. I discovered the fuse box for the trailer in the back and it’s missing a part that the fuses and 1 relay would be in. A relay and 3 fuses for the trailer plug aren’t even there Looks like the fuse box (block) has an additional piece missing that the trailer hitch has to have. Any help would be great

Those 4 empty fuse slots are for the left side fuse blade. The slots for the other fuse blades are even there. Must be a separate fuse receptacle I have to install

 

What is the issue you are having?

 

no power to the trailer light plug. The receptacle that 3 fuses go in for the power isn’t there Dealership says I have to buy a whole fuse block for $500. Then I guess I’d have to get it installed. Has to be a better way. Is this typical with a vehicle that comes with a towing package?

 

I just checked 2 of the 3 164's I own with factory towing packages. None of them have the fuses you are being told need to be there (mine look just like your picture.) You are being told wrong information. Your rear SAM may have water/corrosion causing your problem, or the dealer doesn't know you need a special adapter to make your trailer lights work; read this thread:
https://mbworld.org/forums/gl-class-...n-problem.html

 

All lighting for the trailer plug comes from the rear SAM. If your lights aren't turning on, there are several possible culprits.

The first as mentioned is rear SAM damage.
The second is your car isn't programmed for trailer recognition.
The third is your trailer has LED lights and it doesn't like that. You will need an LED trailer light adapter.

The only connections to the fuse box for the trailer plug are the 12v trailer accessory power and trailer brake controller.