Initial code using Drupal 6.38

misc/form.js

@@ -0,0 +1,9 @@
+
+Drupal.behaviors.multiselectSelector = function() {
+  // Automatically selects the right radio button in a multiselect control.
+  $('.multiselect select:not(.multiselectSelector-processed)')
+    .addClass('multiselectSelector-processed').change(function() {
+      $('.multiselect input:radio[value="'+ this.id.substr(5) +'"]')
+        .attr('checked', true);
+  });
+};