Now head to the old Tiger II Tank outside. You need to interact with it on four places to fix it with the Dark Aether wrench. After doing so, a zombie will crawl out of the hatch on top of the vehicle: Kill it!
Now locate a nearby Crafting Table where you need to craft a Semtex Grenade. Throw this grenade at the little hatch where the zombie just crawled out of. This will cause the Tank to fire a missile at a tree located at the Crash Site. Head over to this tree and you'll be able to pick up a 'Decontamination Agent'.