I can easily explain why this happens with overseas stuff as I am in the business of manufacturing aftermarket automotive parts overseas and selling them in the USA, and also why you call these manufacturers and no one knows anything. The issue is simple, most of this stuff is just copied off a USA manufacturer. You find a product that has huge profit margins and good sales, you buy said product from the original manufacturer, you send it to your overseas broker who finds a manufacturer who can copy it, or who already makes some or all of the parts you need and pieces it for you. You never really end up with exactly the same thing, but something very similar that will work. Quality is usually decent, and it will fit and work, but there will be no instructions, as you do not want to copy the original instructions (and you really can't as it is not 100% the same), and no one there has ever installed this thing to make your own detailed instructions. This gets sold at a good discount from the original kit made in the USA. You can't call these companies and ask them how to install this kit as they have no clue, as they did not design the kit and they have never installed one, they just copied the kit off some other manufacturer. Eventually these companies get to the point where they can help you a bit with the install, as after years of dealing with customers having problems they learn from these customers how to install the kits...
