I don't recommend trying to communicate across modules on a page, that is a very fragile approach because there is no guarantee that everything needed is on the page and as a matter of separation of concerns a module should not have any knowledge of other modules.
If you want a value in a module to change as a result of a change in a dropdown then that dropdown should be self contained inside the module. Modules/Features should be self contained and have no dependency on other modules being on the page.
The only communication strategy that I recommend is one that does not require specific knowledge of another module nor knowledge of what else is on a page. Like a Feed Manager can consume the rss feed from a blog module without any knowledge about the blog module other than its feed url. So there is communication by means of a feed but not communication somehow within the page and no knowledge of what else is on the page is needed.
Hope it helps,