The E7A that I'm installing the DCC decoder in is a remotored Model Power. I removed the Model Power motor and installed a Canon EN22D double shaft motor. The EN 22 motors locked rotor current draw is well under one amp so any decoder will easily handle this locomotive.
The eBay harness doesn't come with the correct color wire to the correct connector contact. That is an easy fix by simply removing the connector contacts by releasing the nylon lock for each contact using a straight pin to pry up the nylon strip above each contact and pulling on the wire. Then the contact can be pressed into the correct connector hole.
As you can see in the picture above the Digitrax DHWH harness has much smaller wire but for HO scale locomotives the larger wire won't be a problem. In this picture the wires were removed and are now in the proper holes in the 9 pin socket.
Next I cut a 1¼" wide strip from .060" thick Styrene to make a speaker enclosure to fit the Model Power shell.
The hole is one inch in diameter, a Greenlee metal punch works great for making holes in Styrene sheet.
Here I used a reamer to make a slightly larger than ⅜" hole to allow for the bolt in the Greenlee hole punch.
Here the hole punch is doing it's thing.
With the hole cut everything used is in this picture. The speaker and the speaker wire with the JST (1.5mm ZH) connector harness were purchased off eBay.
The end piece is glued in place as is the speaker. I have soldered the speaker wire and connector in place and cut a detent for the wires.
I added a small piece of Styrene as a support to keep the enclosure level while the glue was drying.
Here the enclosure is finished and ready to produce some great locomotive sounds. I have installed speakers in six of my Model Power shells. Only four Model Power chassis have Canon EN22D motors but I have three pairs of E7A&B shells. It's nice to have a spear set of shells.
Now on the the Decoder install:
I used .040" thick sheet Styrene to make the Decoder mount and also to protect the wiring from getting tangled up in the drive line.
I used Goop to glue the eBay wiring harness and 9 pin connector to the Styrene.
I dressed up the wiring to keep it neat then used Goop to hold the individual wires in place.
Any 9 pin Decoder will easily plug in to the connector and the fit is tight enough to support the Decoder, no glue or double sided tape required.
The picture below has a Digitrax DH123 installed to show what it looks like with a Decoder in place.
I soldered a female 8 pin micro connector with 1.5K Ω ¼ watt resistors for the Function outputs. I soldered a female two pin micro connector with a 150Ω ¼ watt resistor paralleling the green F1 wire that will be used to operate the 5 volt HK4100F relay in the dummy E7B passenger light power unit that controls the LED lighting.
This Model Power Chassis is going to be used as a E7B but because I wired all the headlight Functions to the 8 pin connector an E7A shell will also fit on this chassis.
Another modification to the Model Power Chassis is a power connector.
I used two single pin male micro connectors back to back for a master power connection. I soldered the red DCC harness wire to the soldered connection between the two connectors. I used single pin female connectors on the wires from the trucks that plug into the male connectors. That makes removing the trucks simple, no unsoldering necessary.
That pretty well finishes the Decoder installation. I installed a MRC1730 Sound Decoder in this Model Power E7, the Prime Mover sound is EMD 567.