Trying to add a type modifier to user created type

Hi *,
I have the following problem:
I wanted to add a new type that supports modifiers (comparable to numeric/varchar). I succeeded in adding the type modifier functions to my new type. These methods are called and the modifier is set. However the modifiers are not applied here. I don't know exactly how to get the modifiers to be applied to my type. I looked into the source code of varchar/numeric and a function called the same name as the type is added in there that gets called with a value of the particular type and the typmod, respectively. But I don't know, how to register this function properly in order to get called, when sth is to be stored in the database. I also set up such a function and registered it in pg_proc (just called create function and made it public like this). My problem is that I don't know how to tell the database system when to call my function. I don't see any possibility to do this here. Even reading the code of numeric/varchar didn't help, because it just works in there. Where do I have to set up that this function is to be called when the modifier is to be applied? I thought, there would be something "magic" that just calls the function named as the given type in order to apply the modifiers. My modifier function, however is never called. The type modifiers (typmodin and typmodout) are called properly when requesting a description of the particular table or setting the modifier. However, unfortunately, it is never applied to my column. How can I achieve this/What do I have to do to get it to work?

Best regards
Carsten Kropf