Here are a few of the decoder installs I've done.
This is a fairly easy install, but Stewart uses yellow LEDs for the headlights and I don't like them, so I replaced them with Miniatronics Yelo-Glo soft white LEDs. They came out way too bright so I ended up swapping resistors as well. The LEDs make contact with tabs on the circuit board, so I merely bent the leads of the new LEDs to match the old ones, as seen in picture #2. Picture 3 shows the board with 1K resistors replacing the original 470 ohm ones. I also touched a bit of solder to each clip from the truck pickups and motor leads, as they are rather loose. Picture 4 shows the decoder and wires secured with kapton tape.
This is probably the easiest install you can possibly do.
For this install I hard wired a TCS T1 decoder. By hard wiring the decoder in place, I didn't have to spend the extra $5 or so for a harness, and it works just fine. The T1 decoders come with a 9-pin JST harness so it is still possible to remove the decoder.
This loco comes with a DCC socket, but there is a wiring fault. One motor terminal is grounded to the frame. The wheels can possibly contact the frame if the loco derails for any reason, and if it should, it would blow the decoder. The solution is to run a new dedicated wire from the offending motor brush to the DCC socket. Link to external photo album on Webshots of this process: Click Here
This loco also comes with a 8-pin DCC socket. You COULD just plug in a decoder, and not even worry about the light bulbs - the stock circuit board drives the lamps through an appropriate dropping resistor. However, due to the circuit design, the lamps are dim. I decided to eliminate the problem completely and switch to Golden White LEDs. At the same time, I completely remove the stock circuit board and hardwired the decoder, a TCS T1. Link to external photo album on Webshots of this install: Click Here