You have to write a Plugin.
If you want do show the result in the postbit, use the hook "postbit_display_complete" (Position 1-5, just try)
Then enter a code like this:
(gars5 = Costum Field Nr. 5, you might change the number)
Code:
if(isset($post[gars5]))
{
$showatgooglemaps = "http://maps.google.com.au/maps?f=q&source=s_q&hl=en&geocode=&q="
. $post[gars5];}
}
Now, all you have to do, is insert the variable $showatgooglemaps in the templat gars_pb at the position you want it to appear.
If you want to use it in the threadbit you have to use these values:
Costumfield = $thread[gars5]
Hook = threadbit_display
Template = gars_threabit
Good luck!
best regards,
Wizzy