Hudson Valley Märklin Club Technical Page
How to install a 60904 in a 3048
The yellow arrow points to a special "S" shaped clip that must be made to hold the top left corner of the decoder. Also note the notch that was cut in the lower left corner of the motor shield to clear the rearmost driver. Additionally, the motor shield was notched slightly under the new clip. Finally, note that all the wiring is kept as flush to the motor as possible to avoid clearance/short circuit problems in the cab.
Speaking of clearance in the cab, I added a piece of electrical tape to the inside of the cab body, (which you can't really see because it's black tape on a black body,) it's right where the yellow arrow is pointing.
Another thing to note is that the wires from the decoder must go over the pillar that was formerly the mounting point for the reversing unit. If the wire gets beside this pillar, it can get mashed into the driver.
The finished project. The analogue smoke unit was retained, an it works very well. Those keen eyed observers out there will also note the 3087 in the background with smoke, working headlights, and 60902 decoder with Hamo magnet. Stay tuned for some more details on that conversion.