After reading your attempt, without looking at it closer, I would try to wire it this way, but at your own risk of course as I don't have the board to hand to examine.
On one of the centre bogies, solder an additional feed from each pickup, run these direct to the appropiate wires on your decoder (power + & Power -)
Assuming the lighting circuit isn't a "Circuit" eg if you follow the tracks from where you made the cut, follow it to the last LED, do the circuit/tracks carry on or end in mid air?? If the latter then solder the two wires to the function part of the decoder. The colours I have forgotten at the moment however i'm sure you know what I mean.
That will only work for 1 coach, so you would need 2 decoders if it works!
I don't have the guts to do any of the above, I bought an on/off switch for an isolated section of track for my E-Train!