Pull the fuel tank away from the chassis. Remove the two screws from the underside of the chassis, one inside the fuel tank and the other near the bogie. The centre hood is then removed. remove the 21 pin DC plug and plug in the decoder, taking care with the mounting orientation.
Two, 16x35mm speakers (8 ohms) can be wired in parallel, but one speaker interferes with a centre hood screw during reassembly. If this presents a problem, replace that speaker with a 16x25mm speaker (4 ohms), such that both speakers would need to be wired in series (16 ohms).