[TYPO3] Adding new columns to a palette

[Rick Tokke]

I have written an extension to add some extra input fields to a extension.
Using:
1. t3lib_div::loadTCA
2. t3lib_extMgm::addTCAcolumns
3. t3lib_extMgm::addToAllTCAtypes

This all works fine. But by default the new fields are placed on the 
bottom of the form.
I want to add them to an existing palette. I can't find how this should 
be done in the documentation. By 'reverse engeneering' some other 
extensions and the typo3 api I found some possible solutions:

I added the following line after the 3 lines above in the ext_tables.php
$TCA['tt_products']['palettes']['2']['showitem'] = 
str_replace('highlight', 'highlight, my_ext_column', 
$TCA['tt_products']['palettes']['2']['showitem']);

And i tried changing addToAllTCAtypes by adding 
before:highlight,--palette--;;2

t3lib_extMgm::addToAllTCAtypes("tt_products","my_ext_column;;;;1-1-1", 
"", "after:highlight,--palette--;;2");

Both have no result the field is still on the bottom of the page.

Any pointers how to solve this issue?

Gr. Rick