I use the Ngineering.com MARS Simulator for MARS lights. They have a 3-LED version that is as realistic as you're going to get. I typically use the single LED model, it does a very good job of looking like a real MARS light. I'd run the headlight from the TMCC package as it will then be directional. You can tap into that to run the MARS light whenever the headlight is on, or do something fancier. Here's a way to turn the MARS light on only when the locomotive is in motion. I sold kits for these a year or so ago, I have one kit left.
Here's a sample application that performs automatic cab light control, Rule-17 headlight dimming, and automatic smoke volume reduction when you're stopped.
