Bachmann N scale EM-1 wire 3 for stock DCC decoder. What's it for?

[gatrhumpy]

On the stock Bachamnn DCC decoder for the N scale EM-1, there is an extra wire that is not on any of the other DCC decoders. It's wire 3. What's it used for? I want to replace the stock decoder with a tsunami, but don't want to fry it.

[gatrhumpy]

No one?

[gatrhumpy]

I found the isometric diagram for the engine. Apparently wire three is an AUX function. In this particular engine, it's a dimming function for the headlights. Never saw that before in a stock Bachmann decoder. Pretty cool.