Make sure you have a B unit that has an LCRU will take the connection. About a year ago, I tried the same thing with a Santa Fe B, and it would not connect. Here's the report I got then from ERR:
I looked up your Santa Fe B 6-14517. Unfortunately it has one of the older LCRU units in it . There are several versions of LCRU's that Lionel has made over time. ERR RailSounds works with some LCRU's and not others. If your RailSounds now works in Conventional mode, then it must be getting 3rd rail power and common ( outer rail ) . The diagram I found does not use the Red wire of the white four pin connector, it must be 3rd rail power for your RailSounds to work in Conventional mode. The blue wire is serial data in the diagram that I found , so it goes to the correct place on the ERR RailSounds commander board. But I don't think that the ERR RailSounds gets the serial data in the correct form to be able to use it.
I was never able to get sound control until I sent the unit in and had the LCRU replaced along with the sound installation. Boxcar Bill, here on the forum, did it and did a great job.