I'm not sure about 300E but I'm guessing in the same place as on a 190E, you'rs is just a bigger brother.
It should be on the fuel distributor, it is a plastic box, with two wires going into it, and is on the side of the fuel distributor most likely in the back. You can see a pic of it on FastLane for comparison.
If you want to get the current, you have to either get a harness from MB for it, or simplay cut one of the wires and put in your MM in it's place.
Remember basic electronics here, current is measure differently then voltage.