One question is: why solder wires to the Plux 22 decoder ? one has a loco which has a Plux22 slot to go into., it would be a great benefit to see the decoder and its wires soldered to it so one can ascertain what's going on.
Unless you find out which pin belongs to which solder pad you're in trouble and the pins don't match up to the same location as the solder pads.

John

Ok, the wires you've connected are right but the coulor scheme is totally wrong. if you go with a NRMA color scheme and using ESU decoders I would stick to the
ESU coulor scheme and not the Märklin colour ,

red wire = middle track
black wire = outer track
white wire = front lights
yellow wire = rear lights
orange wire = motor rear
grey wire = motor front
blue wire = common (DC)
green wire = Aux 1
purple wire = Aux 2

it would also be advisable to use insulated (lacquered) copper wire available from Jaycar No.: 13, it comes in a roll. secondly have you got a Lokprogrammer and a decoder tester ?

buying electronic parts for locos isn't easy and one must consider how to program decoders, this is why it had taken me several years to decide which system to go with and I choose ESU but this is purely a personal choice and in my case it was the lokprogrammer what made me buy the ESU system including at the time the decoders either lokpilot or sound decoders.
Having said that how can we solve your problem ?
so you have tested the yellow wire and the blue wire with a multimeter and there is no connection between them ? and you still have a buzzing coming from the decoder ?
I'll wait for your answer.
and by the way what type of loco are you putting the Plux 22 decoder into it ? I don't like the position of the decoder are you sure no electronic component is touching the metal frame ?

Normally with all ESU decoders you can fit these decoders to the previous existing solenoid reverse unit and buy a back plate and place the decoder there with a double sided adhesive tape.
your confusion also could arise from: there is no short or connection between these 2 wires but an electronic component on the decoder touching the metal frame may not show this with a multi meter but registers as a short or buzzing as you connect the decoder to a power supply.
if you go to the topic: on my workbench you may find under river6109 helpful hints how to convert a loco from analog to digital.

John