To create a product link in showcat, $whichcat for the thumbnail code must be:
$whichcat == -1 || $whichcat == "most_views" || $whichcat == "latest" || $whichcat == "random")
You're passing $pcat from showcat.php, which is the ID tag for the product category. Therefore the code as is can't produce the link types it's meant to.
Look at your own install here: http://www.omegatron.net/rp/showcat.php?cat=53
Now look at the URL link on the thumbnail product image.