For the guys that have their ecm's mounted inside the car/truck. What grommets are you using to fill the void? It needs to be large enough that the connectors can pass thru but needs to be sealed so the tracks dont give ya **** for being able to see light through it.
Summit sells a nifty one that comes in several different sizes and one of them is perfect for what you are trying to do, grommets is actually 2 pieces that seal and protect the wiring very well, I don't have the part number but just go to the summit website and put in " seals it grommet" or I know the brand name is "Seals It" for the keyword search, GL
Summit sells some split grommets made by a company called Seals-It. They are the cats *** as far as I am concerned. I have a few of them on my car for various reasons and I love them. They are not cheap, anywhere from $15-$30 each for different options. I think they are a great product, check them out.
You could always take the wires out of the PCM connector, run each wire through the hole one by one, and then reconnect them back into the connector on the other side of the firewall.
At first it might sound like a huge PITA, but it's really not that bad. Once you get the hang of taking the pins out of the connector, the work will go quickly. I would also go to home depot and buy a couple sheets of the wire number stickers to keep track of the wires. Or you could scrounge up an extra set of PCM connectors, and plug the wire into the new connector as soon as you take it out of the old one.
Your finished product will be great. You'll have a hole that's no bigger than it needs to be, and well protected by a grommet. I did this with my DBW pedal harness and another 10-pin harness and would do it again in a second.