Changed all the scrap in the config to 200, saved, reloaded the plugin, it returned back to default.

can you post config  :)

I don't have it on the computer I'm on at the moment, but the config is the same as default with all scrap changed to 200.

Generally this happens when the configuration format was not edited correctly.

Okay... at that computer now.  Here is my config:

{
  "shredderConfig": {
    "shreddable_pickuptruck": {
      "vanilla": {
        "scrap": 50,
        "metal.fragments": 50
      },
      "modified": {
        "scrap": 200,
        "metal.fragments": 200
      }
    },
    "3module_car_spawned.entity": {
      "vanilla": {
        "scrap": 125
      },
      "modified": {
        "scrap": 200
      }
    },
    "2module_car_spawned.entity": {
      "vanilla": {
        "scrap": 75
      },
      "modified": {
        "scrap": 200
      }
    },
    "4module_car_spawned.entity": {
      "vanilla": {
        "scrap": 125
      },
      "modified": {
        "scrap": 200
      }
    },
    "snowmobile": {
      "vanilla": {
        "scrap": 50
      },
      "modified": {
        "scrap": 200
      }
    }
  }
}

I've changed all "scrap" to 200 and saved, but when I reload the plugin, it changes back to the config you see here.

Your config looks fine to me, are you also changing the scrap under the "vanilla" section to 200 too? The vanilla section of the config is only for reference to see what the default values in rust are. You only need to change the "modified" section of the config, save and reload the plugin.