Radio Button Control

I have three radio buttons grouped together:
<input type="radio" name="MODEL" value="V1" onclick=Enable() checked>Model
V1<br>
<input type="radio" name="MODEL" value="V2" onclick=Enable()>Model V2<br>
<input type="radio" name="MODEL" value="V3" onclick=Enable()>Model V3<br>

And a text field that is populated from a popup window option selector:
<input type="text" name="FABRIC" size="40" maxlength="40" value="" READONLY>

When one of the radio buttons is selected I need to clear a text field
depending which radio button was previously selected.
For example,
If Model V1 is currently selected, and the user selects Model V2, I do not
want to clear a text input field.
If Model V1 is currently selected, and the user selects Model V3, I want to
clear a text input field.
If Model V3 is currently selected and the user selects either Model V1 or V2
I want to clear a text input field.

How can I accomplish this or tell what radio button was previously set?

Thanks,
Dave F.