Can't get hook_uninstall to trigger In mymodule.install i try to implement a hook_uninstall call, but it never triggers. What am I doing wrong?

function hook_uninstall() {
echo "uninstalling";
...more stuff...
}
First off, the implementation of a hook never has a name starting with hook_; you always replace hook_ with the machine name of the module, followed from an underscore. Similarly, the name of the file containing an implementation of hook_uninstall() must be the machine name of the module followed by .install.
So, if you are literally calling the function hook_uninstall(), and mymodule is not the machine name of your module, Drupal will never consider that function a hook implementation.

Second, if you are using echo to check if the hook is being invoked, you will not get any output. Use watchdog() (Drupal 7) or \Drupal::logger($module)-&gt;error($message) (Drupal 8) inside a hook, if you want to verify a hook is invoked.

Third, hook_uninstall() is invoked from Drupal only when a module is uninstalled. There is no other way to be sure Drupal is invoking that hook, if not installing and uninstalling the module implementing it. In Drupal 7, don't get confused from disabling a module, and uninstalling it: Disabling a module doesn't cause Drupal to invoke hook_uninstall().