hook location for header footer (in thread) plugin?

[fabioski]

What hook location for plugin which has to working in HEADER AND FOOTER template in each thread?
Which hook location have I to choose?
I try with showthread_complete and showthread_getinfo but it doesn't work

The plug-in generate correctly the variable:
$testo_adv_header .= "$row_adv_header[adv_text]";
and I stamp it on site correctly:
echo "$testo_adv_header";

But in the template the variable does not appear!
<td>$testo_adv_header</td>
</tr>
</table>

And for footer template?

--------------- Added [DATE]1221900556[/DATE] at [TIME]1221900556[/TIME] ---------------

Is it not possible to make a plugin for Header template?

For example:
in my plugin i create this var:
$var1 = "<p>TEST PLUGIN</p>";

if I put in HEADER template this var:
$var1

I not read "TEST PLUGIN"

Which hook location Have I to use?