Retrieve the Decontamination Agent

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'.