Spiff,
Here is the basics of what you have to contend with.
The way the decoder works, it takes the DCC power signal from the track and converts it a controllable DC.
What you want to do is isolate point 4 from point 3. That way you can connect the orange and gray wire to point 4 and the red and black wire to point 3.
I'm not sure how intrusive you want to go. I like to install in a manner that I can go back to DC if need be.
Here is how I would do the install. (this is based on what I see from the photo which is limited.)
Isolate the motor by
Next use this trick the little brass flat springs comes in handy.
You can use these brass clips and fold them around the edge of your base bringing the connection topside.
This is how I would do it but there are other ways. You could drill or just solder the wire onto the motor point it all depends on what you want.
One thing I cannot stress enough is to isolate the DC motor leads and make sure you don't have any shorts.
Make sure that your soldering is only a few seconds you dont want to over heat the decoders components.
Good luck!!
Inobu.
