Radiator Drain Screw Location on C280?
 

I have easily located the red radiator drain plug on my C280 (1995), but I cannot locate the ELUSIVE engine block hex nut (apparently the second bleeding point.)
I attempting to flush my radiator system.

Can anyone please help explain where I can find this screw? I can't flush my system without it.

Passenger side?

Apparently it is somewhere on the passenger side and can only be seen from under the car, which makes sense...

thanks,
mike