I feel your pain. Harnesses are a big issue no matter what make the truck is and remember they all share the same emissions components... Even Kubota.
The particulate filters can be cleaned but the machine costs 50 grand and the residue is hazardous waste so all the dealers just change them out and send the cores to the respective engine builders to get rebuilt. It's a racket.
The one boxes don't fail (unless the truck gets hit on the passangers side). What fails is the weldments and again, most diesel shops lack the equipment to fix them. Detroit Diesel provides me with all the fittings to repair them. I'll probably get back into doing them again this year (I had to quit because of my cancer issues). I fell into it by accident. The guy I hunt with is the service manager for the local Freightliner / Western Star delaership that I retired from and he asked me if I was interested and I was because the rework is real simple. Once in a while I have to weld an internal baffle but usually it's the sensor bungs.
Not knowing what brand of trucks you have but living in a salt enviroment in the winter, I'd pull all the Molex connectors apart that you can get to and put di-electric grease in them all. Modern harnesses aren't anything like the ones of old. Insulation is thin and so are the wires. Good technicinans don't probe wires today, they use a multimeter. Tell the service manager to take his 12 volt light away from him, he's costing customers a ton of money. You compromise the insulation and it's a time bomb. Salt gets in there corrodes the wires, causes high resistance and the motor derates. Everything today is done with resistance values.
My friend also owns 40 tractors, all Tier 4 final Freightliners leased to Fed-Ex with team drivers and his average monthly service bill is 45 grand a month. He makes money but spends money too. That is routine maintenance, not breakdown money.
Don't want anything to do with them. I sit back and listen and smile.
Was always a company driver, always drove extended hood double bunk Stars with Cats of the 3406 variety. They smoked, just like my Kubota's smoke. If it don't smoke, I don't want it.