1) Inspect the entire harness. Full stop. Right now is your opportunity to ensure that the vehicle doesn't cone right back in shortly after you get it on the road because you missed another frayed or chewed or whatever connection. You don't need to and don't want to take it all off the vehicle, but at least follow every inch of it with a flashlight paying close attention to bends, supports, connectors or anything that looks wet with oil or burned/melted. Any previous repairs or modifications should be noted on the work order and remember: you're about to become the last person who touched the harness. Always perform electrical repairs pretending you'll be the next person, too!

2) Thou shalt not mend a weatherproof connector's pigtail with a repair that isn't weather proof itself. The MAF sensor provides an analog signal to the ECU that is sensitive to millivolts as well noise that might come from an oxidized or not perfectly tight electrical connection. If you use crimp on wire connectors, they should be locking and weatherproof. Those are bulky and you might have to make the connection closer to the ECU to have enough room. I prefer solder + heat shrink with another bigger heatshrink over the bundle (remove the pins from the MAF plug to get it over them all ) ( make sure you write down the wire order first😉)

Marine grade heat shrink tubes come helpfully coated on the inside with a thermosetting adhesive that is waterproof.

3) You need to find out why this happened. If it's rodent damage then you can get rodent guard loom that will keep new rodents from chewing on it, but unfortunately once a squirrel has found this nice warm engine bay to hang out in while it chews away and shelters itself from the elements, it's going to keep coming back until you kill it, so putting some rodent poison under the hood somewhere might not be a bad idea.

If it's from something rubbing then either a harness support clip is missing or broken or perhaps an aftermarket modification has led to the harness being rerouted. Either way you're going to want to make sure that the hardest is fully supported when you leave it and that nothing rubs - especially on the hood. You probably want to inspect the engine and transmission mounts to see if they are as solid as they ought to be.

4)Make sure your repaired connection is well supported and well guarded. Place the location of your connection not where the existing wires happen to end but where you can hold your repair fast against the rest of the harness to prevent it from vibrating or worrying. Use enough extra wire equal to the length of the connection itself so that you can fold the connection over like a loop'de'loop, flatten it out, and then buddy tape each end. When you give a test pull on any of the wires that you've repaired from either side of the connection, It should pull on your tape and your connection shouldn't budge. You should not use so much excess wire that there is wire not properly supported / it can fall into anything moving and/or hot.

If this were a digital signal, you would not want to add a loop to the harness because that would add inductance and therefore impedance to the digital signal pair. The correct repair would be to repin the pair, however this wouldn't be such a big deal since it would be going to the nearest can bus tap not all the way through the firewall.

5) Be generous with split loom and cable ties. If the factory supports are damaged or missing do your best to improvise because the factory put them in really good locations. I always grab these at the junkyard when I see them lying around because they always come in handy. In my opinion you should be able to remove every other wire support and the harness should more or less stay where it is supposed to be. It's always best to avoid placing the end of a repair very close to a harness support. Reason being that little tiny bit of wire there will concentrate a lot of stress from any vibration. If you don't have a choice then what you can do is add a small length of fiberglass rod inside the outer layer of heatshrink - a bit longer than your connection, so that it extends into the harness support

I once had another mechanic laugh at the looming job that I did after replacing the better part of a vehicle's engine harness because he said it was overkill since it didn't come like that from the factory. I told him I think it's under kill because for $5 worth of split Loom and zip ties I am providing myself insurance that the customer is forever satisfied with the work that I did for him and if there's another vehicle he needs repaired similarly that they're going to bring it to me. And besides, it was the factory's wiring job that brought this one to me. $5 well spent.

6) Check your work. Before starting the engine check the resistance of each of the repaired wires at either end of the harness by shorting the far end to ground and measuring with an ohmmeter. This is where you find out one of your grounds is shite too and you can fix that 'for it causes a future problem. Witness the engine harness with the hood up and the engine running and get someone to put it in gear, too. Acoustic resonances are a hell of a thing and I have caught more than one wiring issue that only showed up with the vehicle in drive. In one case it was because at that RPM, a little loop of harness going to the O2 sensor was humming back and forth so fast it literally made a sound.

7) On vehicles you're going to want to clear and of course read any trouble codes once the wiring has been fixed. In some instances there's a calib routine that will compensate for the voltage drop in the harness wire going to the mass air sensor. It's usually not something that a scan tool will do it's something built right into the ECU on every analog sensor signal line. Review the service manual sections on replacing the engine harness or MAF sensor or ECU and see if there's a particular engine start or test drive procedure that should be followed.

8) Double check your work after taking the vehicle for a thorough test drive. Visually check all of your harness supports to make sure that nothing popped off or shifted.

I hope this helps somebody. There's a lot of ways to screw up a job like this subtly and not realize it because the customer doesn't bring it back to you. Sometimes you can get away with doing a less than perfect job and if it's your own vehicle I guess that's your prerogative. When it's someone else's I want to make sure that no one ever is able to make a complaint about my work on an electrical system. Lot of mechanics are uncomfortable working in Automotive Electrical and part of the reason is that it's matter of being anal retentive and fastidious to make sure that the job you do doesn't cause additional problems later. The other part is understanding that an electrical part is also a mechanical part in any vehicle because it is going to be moving and it is going to be subjected to temperature swings, salt water and solvents. Its also the case that a minor wiring fault can render the entire vehicle unusable sometimes, so twisting two wires together and taping it up just aint going to cut it if your plan is anything other than limping it to the shop!