Jake brake wiring

Hello Folks. I realize this is an older thread. I have a 97 Freightliner FLC with an N14 Celect Plus. The truck came without Jake Brakes. I found a good set of 3 stage brakes and had a mechanic install them. From the very start this project has been a goat roping. The original mechanic mucked it up. The jakes were either on or off. The only function was the first and second level. If you had them turned on and stood on the throttle, the third level came on. He tried or said he did several different fixes with no results. He only installed one three stage switch.

I picked the truck up and you could hear an exhaust valve squeaking. The Jakes sucked and we tried to make do.

I talked to another mechanic, he had a great rep, so I took it to him. He could not do anything with it. He took the heads off because it was suppossed to be "pushing water". The heads were sent to a machine shop to be checked. Everything was ok with the heads, they were reinstalled. A new oil cooler was installed, and some other items were fixed. He did a very good job with the repairs, but he could not do anything with the brakes.

I read on this thread about a wiring harness for power and three position switches. Following that advice, I pulled the instrument panels and could not find them.

I am looking for wisdom from you folks to fix this mess. We are a dedicated carrier for a lumber company and transport green and kiln-dried throughout the mountains of North Carolina, Tennessee, Virginia, and West Virginia. We have an alumalog system we use to haul logs back down these mountains....Fancy Gap and Black Mountain, so jakes are a must.

Thanks in advance for any assistance or wisdom that you can lend.