Choosing content depending on content of other element

Hi list,

I think I'm stuck.
I've been trying on the following problem, but I can't figure it out.

I've merged two xml-trees. That's fine so far.

No I have the nodes /root/output/data/product_id and order/product

Depending on the product_id output, I wan't to choose
the equivalent node

/root/order/product/@id='xxx'/content/@group

Example:
When I have output

So when /root/output/data/product_id  => 404
I want to output all the equivalent
/root/order/product/id='404'/content/@group

Beneath there's the xml-tree.

Beneath is also the current version of the xsl-stylesheet. I've tried so far
(without) success:
Why does it not work?

Regards Jochen

xml
===
<?xml version="1.0"?>
<root>
   <output>
      <data>
   <ordernr>9026</ordernr>
   <productnr>PD1X404</productnr>
    <product_id>404</product_id>
      </data>

      <data>
         <ordernr>9026</ordernr>
         <productnr>PD1X405</productnr>
         <product_id>405</product_id>
      </data>
   </output>

   <order nr="9026">
     <product id="404">
        <content group="9026-P404-M402-1">
          <shops_booked>4</shops_booked>
            <status_in_lang>in Bearbeitung</status_in_lang>
   </content>

         <content group="9026-P404-M402-2">
          <shops_booked>2</shops_booked>
   <status_in_lang>Online</status_in_lang>
         </content>

      </product>

      <product id="405">
        <content group="9026-P405-M402-1" />
   </product>
 </order>

</root>

XSL
=======
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
   <xsl:output method="xhtml" version="1.0" encoding="UTF-8" indent="no"
omit-xml-declaration="yes" doctype-public="-//W3C/DTD XHTML 1.0
Transitional//EN"
doctype-system="http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
standalone="yes" media-type="text/html" />

   <xsl:template match="/">
      <xsl:apply-templates select="/root/output" />
   </xsl:template>

   <xsl:template match="/root/output">
      <xsl:call-template name="line" />
   </xsl:template>

   <xsl:template name="line">
   <br/>
   <xsl:for-each select="data">
            <br/><br/><b><small><u>Produkt</u></small></b><br/>
            <xsl:value-of select="productnr" />
            <br/><b><small>Content-Gruppe</small></b>
           <xsl:apply-templates
select="/root/order/product[@id='/root/output/data/product_id']"/>
  </xsl:for-each>
   </xsl:template>

   <xsl:template
match="/root/order/product[@id='/root/output/data/product_id']">
     <br/><xsl:value-of select="content/@group"/>
   </xsl:template>
</xsl:stylesheet>

xml
===
<?xml version="1.0"?>
<root>
   <output>
      <data>
   <ordernr>9026</ordernr>
   <productnr>PD1X404</productnr>
    <product_id>404</product_id>
      </data>

      <data>
         <ordernr>9026</ordernr>
         <productnr>PD1X405</productnr>
         <product_id>405</product_id>
      </data>
   </output>

   <order nr="9026">
     <product id="404">
        <content group="9026-P404-M402-1">
          <shops_booked>4</shops_booked>
            <status_in_lang>in Bearbeitung</status_in_lang>
   </content>

         <content group="9026-P404-M402-2">
          <shops_booked>2</shops_booked>
   <status_in_lang>Online</status_in_lang>
         </content>

      </product>

      <product id="405">
        <content group="9026-P405-M402-1" />
   </product>
 </order>

</root>




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