Так чёле?
<!doctype html>
<html>
<head>
<style type="text/css">
</style>
<script>
function newCount() {
var o=document.getElementsByName('selectedcars')
var i
if (o[0].checked) {
o=document.getElementsByName('kategory[]')
for (i=0; i<o.length; i++) {
o[i].checked=false
}
}
}
function newCountCh(Obj) {
if (Obj.checked) {
var o=document.getElementsByName('selectedcars')
if (o[0].checked) {
o[0].checked=false
o[1].checked=true
}
}
}
</script>
</head>
<body>
<div>
<input type="radio" name="selectedcars" value="1" checked="checked" onclick="newCount();">Alle Fahrzeug<span>typen</span>
<input type="radio" name="selectedcars" value="2" onclick="newCount();">Ausgewahlte Fahrzeug<span>typen</span>
</div>
<div>
<input type="checkbox" name="kategory[]" value="1" onclick="newCountCh(this);" />Limousine
<input type="checkbox" name="kategory[]" value="2" onclick="newCountCh(this);" />Gelandewagen/Pickup
<input type="checkbox" name="kategory[]" value="3" onclick="newCountCh(this);" />Kleinwagen
<input type="checkbox" name="kategory[]" value="4" onclick="newCountCh(this);" />Cabrio/Roadster
<input type="checkbox" name="kategory[]" value="5" onclick="newCountCh(this);" />Kombi
<input type="checkbox" name="kategory[]" value="6" onclick="newCountCh(this);" />Sportwagen/Coupe
<input type="checkbox" name="kategory[]" value="7" onclick="newCountCh(this);" />Van/Kleinbus
<input type="checkbox" name="kategory[]" value="8" onclick="newCountCh(this);" />Andere
</div>
</body>
</html>