Trying To Use A JQuery Plugin In A Control?

I am trying to use a jQuery plugin in a control. The pages that the control can be on use partial postbacks via an UpdatePanel. I include jQuery and the plugin during the control's PreRender event like this:

ScriptManager.RegisterClientScriptInclude(
this,
this.GetType(),
"jquery",
"/_infrastructure/javascript/jquery.js"));
ScriptManager.RegisterClientScriptInclude(
this,
this.GetType(),
"jquery.customPlugin",
"/_infrastructure/javascript/jquery.customPlugin.js");

The customPlugin jQuery plugin sets up a new function called "executeCustomPlugin". Later in the PreRender event of the control, I use the plugin on an element on the control:

ScriptManager.RegisterStartupScript(
this,
this.GetType(),
"customPlugin init script",
@"$(document).ready(function() {
$('#elementId').executeCustomPlugin();
});",
true);

However, when it executes, I get the JavaScript error:

$('#elementId').executeCustomPlugin is not a function

It would seem as if the jQuery plugin is never executed at all, but I set up window.alerts in the jQuery.customPlugin.js file, and it is indeed being executed.

Is there a way to fix this problem?