Freightliner M2 106 chassis wiring.

I'm new to the semi/industrial truck scene so please forgive my ignorance. Customer attempted to jump the 12 volt system with a 24v jumper and fried a couple of things. The dealer put a new computer in and when that didn't work they suggested to replace the fuse block? Right under the transmission computer. He replaced that but the truck still won't start. All fuses and relays checked. When key is put on the dashboard dings and the gauges do their sequence, turn the key and all you hear is a clicking (relay) under the dashboard. All the wiring diagrams do t explain what the 4 + leads go to and instead of undoing all the plastic shield for all the cables I'd thought I'd ask here to see if anyone had any experience with this type of issue. I'm not sure what to try. I will straight up PayPal someone $100 if they give me a solution that fixes this truck. Im I mobile mechanic that mostly works on passenger vehicles so I'm pretty clueless, today I realized I might want to start working at a dealership for semis to start getting some knowledge because there is money to be made in the commercial sector. Like I said please understand that while I'm not clueless as to how it works I have effectively 0 experience with these trucks. I'd appreciate all suggestions.