When trying to convince managers you have to bring it down
to the fundamental things they work on: time and money.
When presented with a project come up with 2 estimates.
One
based on using modules (be sure to add the additional
time
it takes to install the modules on all the necessary
machiens).
The other based on writing (and testing)
the functionailty that you need
to re-invent from modules.
Let the manager choose. If he doesn't choose to use modules
(almost always the cheaper solution)
he will have to justify to the layer of management above
why he choose the more expensive option. In my experience it is
frequently much more expensive to re-write module functionality
than to use existing modules (1 day vs 2 week projects, etc..).