function AddInboxBack(document) {
var list = document.getElementById('masthead-expanded-menu-list');
var li = list.children[0].cloneNode(true);
li.children[0].href = "/inbox";
li.children[0].innerHTML = "Inbox";
list.appendChild(li);
}
The above Javascript will add the inbox link to the bottom of the list it used to belong to. How you implement it is up to you - I'm going to use a userscript.
Edit: Greasemonkey Userscript:
// ==UserScript==
// @name Add Inbox Back
// @namespace https://www.youtube.com/
// @description Puts the inbox link back.
// @include *://www.youtube.com/*
// @version 1.0
// ==/UserScript==
function AddInboxBack() {
var list = document.getElementById('masthead-expanded-menu-list');
if(list != null ) {
var li = list.children[0].cloneNode(true);
li.children[0].href = "/inbox";
li.children[0].innerHTML = "Inbox";
list.appendChild(li);
}
else {
console.log("Could not add Inbox to this page.");
}
}
document.addEventListener('DOMContentLoaded', AddInboxBack, false);
The javascript is set to load on any page from YouTube.com. It looks up the top list. If it finds it successfully, it clones the first entry/link in the list. Then it edits the link to the inbox and the text displayed to say Inbox. Then it adds this to the bottom of the list. If it can't find it the list, it will put an error in the javascript console, just so you might find what's going on. And of course the event listener for content loaded so it waits until it is sure everything is ready.