How do I change the properties of a DOM element based on a property of an adjacent field?

I'm trying to change the background positioning for a label based on whether or not an adjacent checkbox is checked.

I figured out how to s开发者_运维技巧elect adjacent fields with jQuery and I verified that this selector is indeed working:

$("input[type='checkbox'] + label")

But I can't figure out how to access the checkbox.

Here's what I have so far...

$("input[type='checkbox'] + label").click(function() {
    $(this).css("background-position", $("--adjacent checkbox here--").attr('checked') ? "bottom" : "top");
});

$("input[type='checkbox'] + label").click(function() {
    var $th = $(this);
    $th.css("background-position", $th.prev().attr('checked') ? "bottom" : "top");
});

http://api.jquery.com/prev/

jQuery's prev() method selects the immediate previous sibling if one exists.