1994 FWD Engine + Transmission, Engine Harness, MPI Relay, and ECU into a 1989 C52A chassis and original internal cabin harness setup.
First things first, these pics were taken from an 89 Plymouth Colt with the 1.5 4G15 engine and the factory 5 speed tranny/hydraulic clutch. Your car's wiring may be different if it's a different year model or came equipped with an automatic transmission. I am using the engine,tranny, engine wiring harness, ECU and MPI relay from a 94 FWD Turbo DSM. If you are using a different combination there may be some differences in the wiring, I'm not sure. This works for me on my car, if you attempt this swap and mess something up I will NOT be held responsible! Now onto the wiring.
We'll start under the hood. First, disconnect the (-) negative battery terminal. Now look on the passengers side of the engine compartment and you will see this fuse box.
If you look under it there will be a bunch of wires coming out leading to a connector. The colors are black/yellow, red/blue,teal,a large black/red and a smaller black/red wire. You want the black/yellow one. This wire needs to be ran to the little plug on the starter harness. It's also a black/yellow wire which plugs into the small tab on the starter. This will get your starter to spin once you turn the key to crank the car. Here is a pic of the connector, a little blurry but you get the idea. Note: I used the 4G15's starter harness, not the DSM one. Also be sure to use the three fuse block on the DSM's engine harness. Attach this to the positive (+) battery terminal as it sends power to the MPI relay.
Now onto the interior wiring. Karl (Netninja2001) already has an excellent writeup on this part that he did while wiring up his car. This thread can be found here.
He used a harness from a 90 model and mine is from a 94 so I found a few differences. On the C50 connector, wire # 3 was red/black on his and I found it to be a black/white wire on mine. This was for the ecu's memory. I didn't use the black/white wire but instead I used a red/blue wire from that same connector and connected it to the white/green wire on the Colt's C19 (wire #7) connector.
One other difference is that wire #2 on the C50 connector was blue on his, it's blue/green on my car but I still connected it to the C19 #9 wire. Other than these two differences they are identical so you can just match up the wires and solder them together.
The Colt also has a black 4 wire connector on it's interior harness located under the passengers kick panel by the ECU. Some of the wires from this connector need to be wired into the DSM's MPI relay connector to give the MPI relay it's power. One other thing to note is that I cut the factory 4G15 engine harness and left some length on those wires. Be sure to cut it on the engine harness side. If you choose to just unplug it some of the wire colors may not match up. The wire colors for this connector are black/yellow, yellow/green, a thick black/white and a thin black/white with brown dots.
Take the black/yellow and attach it to the black/yellow on the MPI relay harness. Thick black/white to black/white on the MPI and the thin black/white with brown dots connects to the black/green on the MPI harness. The yellow/green wire goes to the AC I believe so I didn't use it as I chose to leave my AC off.
I just took a razor blade and shaved a little of the rubber off the 3 wires to the MPI relay and wrapped the corresponding wires around them. Soldered all connections and then wrapped them with electrical tape. This pic shows what I'm talking about with the black/yellow wire.
There you have it. Hopefully this will help others when entering this wiring nightmare. It's not too bad, just take your time.