Objects, Callback Functions And Variable Scope

I am trying to convert some of my javascripts into a class and am
running into difficulties which i think are related to variable scope.

Basically I have a constructor function for a calendarInput class that
takes 4 parameters, the first is a reference name/number for this
input. I also have some functions for importing my PHP classes into
Javascript using AJAX (properties only. still trying to get methods
working but that's another story!). The relevant function is called
call_object_method(classname, methodname, createparams, methodparams,
post, callbackfunction). This creates an instance of the specified PHP
class using the parameters in createparams. It then calls the
specified method using the parameters in methodparams. The result is
passed back to the javascript function specified in the callbackfunction parameter (ie the value of xmlhttp.onreadystatechange is set to callbackfunction before xmlhttp.send() is called)

The function i am trying to fix is called show (x,y) which creates the
html for the calendarInput and displays it at co-ordinates x, y.

So, my code is as follows:

function calendarInput(calendarid, target, displaytarget, value)
{
this.calendarid = calendarid;
this.target = target;
this.displaytarget = displaytarget;
this.value = value;

this.show = function(x, y)
{
<!--// bunch of init and prepping code. includes creating
createparams and methodparams arrays //-->

call_object_method("cms_calendar", "getcalendarview",
createparams, methodparams, false, this.showcallback);

}

this.showcallback = function()
{
alert(this);
<!--//code to create html//-->
}

}

I know i've cut out most of the innards of this. This is because I
have already tested these functions and had the calendarInput working
outside of a class, hence am pretty sure that this is ok (plus it runs
to almost 1000 lines these days!!). My problem is that when I call the
show method, the alert on the first line of the callback function
returns the function showcallback instead of (as i was expecting) the
instance of the calendarInput object. Whilst this kinda makes sense I
can't figure out how to reference the Object instead. I have tried
'this.parent' but this returned undefined. I have tried changing the
way i reference the callback function (ie the final parameter of
call_object_method) but no joy.