are you using 003 as loco address? This is usually default address for
new decoders.
If the motor cap is broken - only motor should be affected. Light and
sound should still work.
proper way to solder motor wires is to leave a little bit of the
terminal sticking off of the actual top cap and solder to that bit and
not directly to the brush cap.
May be easier to get new motor than repair the one you have.