Beactive,
чуть изменить label ...
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.popular-category :checked ~ ul {
display: block;
}
.popular-category ul {
display: none;
}
</style>
</head>
<body>
<li id="vehicle_models-29" class="popular-category"><input value="29" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-29" class=""><label for="in-vehicle_models-29" class="selectit">Alfa Romeo</label><ul class="children" >
<li id="vehicle_models-173" class=""><label class="selectit"><input value="173" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-173" class=""> 145</label></li>
<li id="vehicle_models-174" class=""><label class="selectit"><input value="174" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-174" class=""> 146</label></li>
<li id="vehicle_models-175" class=""><label class="selectit"><input value="175" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-175" class=""> 147</label></li>
<li id="vehicle_models-176" class=""><label class="selectit"><input value="176" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-176" class=""> 155</label></li>
<li id="vehicle_models-177" class=""><label class="selectit"><input value="177" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-177" class=""> 156</label></li>
<li id="vehicle_models-178" class=""><label class="selectit"><input value="178" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-178" class=""> 159</label></li>
<li id="vehicle_models-179" class=""><label class="selectit"><input value="179" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-179" class=""> 164</label></li>
<li id="vehicle_models-180" class=""><label class="selectit"><input value="180" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-180" class=""> 166</label></li>
<li id="vehicle_models-181" class=""><label class="selectit"><input value="181" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-181" class=""> 33</label></li>
<li id="vehicle_models-182" class=""><label class="selectit"><input value="182" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-182" class=""> 75</label></li>
<li id="vehicle_models-183" class=""><label class="selectit"><input value="183" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-183" class=""> 90</label></li>
<li id="vehicle_models-184" class=""><label class="selectit"><input value="184" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-184" class=""> Alfasud</label></li>
<li id="vehicle_models-185" class=""><label class="selectit"><input value="185" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-185" class=""> Alfetta</label></li>
<li id="vehicle_models-186" class=""><label class="selectit"><input value="186" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-186" class=""> Arna</label></li>
<li id="vehicle_models-187" class=""><label class="selectit"><input value="187" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-187" class=""> Brera</label></li>
<li id="vehicle_models-188" class=""><label class="selectit"><input value="188" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-188" class=""> GT</label></li>
<li id="vehicle_models-189" class=""><label class="selectit"><input value="189" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-189" class=""> GTV</label></li>
<li id="vehicle_models-190" class=""><label class="selectit"><input value="190" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-190" class=""> Guilietta</label></li>
<li id="vehicle_models-191" class=""><label class="selectit"><input value="191" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-191" class=""> MiTo</label></li>
<li id="vehicle_models-192" class=""><label class="selectit"><input value="192" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-192" class=""> Montreal</label></li>
<li id="vehicle_models-193" class=""><label class="selectit"><input value="193" type="checkbox" name="tax_input[vehicle_models][]" id="in-vehicle_models-193" class=""> Spider</label></li>
</ul>
</li>
</body>
</html>