I had a wiring problem from the factory on my exhaust brake and it took two shops and six weeks to fix it. About 100 shop hours were expended, fortunately for me under warranty. You can look for a loose wire or a fuse problem, but beyond that troubleshooting is very difficult. I would ask if the exhaust brake worked before the steering center mod? A clue like that would narrow down the search considerably. I would look real hard for a blown fuse or loose wire before turning it over to a shop. If you do use a shop you should get the very best you can possibly find because I watched a good one fumble for six weeks. I know just enough to realize that they were doing a good job. I now know that Freightliner in Gaffney could have done the job much quicker and I would have been way ahead making the 600 mile drive.
2014 Palazzo 33.2
Cummins 6.7 ISB, Allison 2100 6 Speed