While I haven’t replaced BLI decoders in any GN S-2s I have done several in New York Central S-2s. Three in fact. Plus two Mohawks, five Hudsons and at least a dozen PRR steam locomotives.
The later runs get more complicated with their additional lighting add-ons. I completely remove any smoke makers and often replace them with round speakers (Scale Sound Systems).
Once you ‘ring out’ the wiring you’re well on your way. “Generally” the two outer pins are track rail A and B (or red for the right rail and black for the left) the next two in are for the motor but you really have to check the polarity as I’ve seen them wired differently, then with a six-pin the next two are the headlight. Use a small 3V coin battery or an LED tester to sniff out which wire is anode and which is cathode.
Newer Paragon 3 and 4 decoders that have the lighting module in the firebox are tougher to sort out as BLI uses common cathode (-) and switches the positive! I’ve had to rewire LEDs on the PC boards on these.
Mohawk TCS ESU Install2 by Edmund, on Flickr
This shows a smokebox-mounted speaker in place of the smoky thing:
J3a_SSS speaker by Edmund, on Flickr
Trial fitting a decoder and speaker in a Niagara tender. To the right is the junk BLI speaker:
BLI_NYC_Niagara-DCC trial-fit by Edmund, on Flickr
Sometimes BLI is nice and identifies all the pin outs for you:
BLI_P3_T1 by Edmund, on Flickr
Hope that helps, Ed