The Life Like SW9/1200 has always been one of the more challenging N Scale locomotive to get a decoder into. No decoder has ever been made specifically for this model so it's been up to the modeler to fit in a wired decoder. I've done a number of these engines and over the years I updated my technique. What is shown below is the most reliable method that I have found.
Next, remove the cab shell after unplugging the center hand rails from the cab. The heavy steel weight may come off as well and that is OK.
Some may feel that inside the cab would be the obvious place to put a decoder. I don't agree with that approach because this is where most of the weight is in this small engine. I have tested these with just the heavy steel weight removed and found that the engine lost much of it's traction and pulling power.
Remove the LED and the resistor from the little PC board.
Gray = 1-1/2 inch
Orange = 1-1/8 inch
All others = 1/2 inch