Another well thought out installation design by SRM. The speaker is already installed so all that needs to be done is to slip the body off the chassis and plug the decoder in.
Simply splay out the body and lift off from the chassis. Remove the 21 pin dummy/DC jumper board and install the ESU LokSound 21-MTC decoder as shown below.
Ensure the 21 pin black header (or yellow "4" on decoder) is facing UP! If it faces down the decoder will short circuit and fail and this mode of failure is not covered by ESU's warranty.