If I copy the file default.phtml from design/frontend/base/default/template/checkout/total/ to my theme folder (design/frontend/my_package/my_theme/template/checkout/total/default.phtml) everything is fine, my changes are applied and I can see them on the front end. But what I want to do is different. I need this default.phtml file to be in a module that can be easily switched off when not needed; to achieve this I need to override the template file but here I got stuck because I couldn't find any reference to this template in checkout.xml
I also tried to modify the renderTotal() function in Mage/Checkout/Block/Cart/Totals.php (I copied the file in my module, didn't altered the core files) and I was able to apply some changes (I needed a colspan=2 instead of 1) but not all of them (the second cell is still being displayed when the colspan is set to 2 while I want only the cell with the colspan=2 to be shown). So the only way I can achieve what I need is to modify the default.phtml file.
So my question is: how can i override the dedault.phtml?