Autohomes didn't 'seal' the front and the back of the hightop, just the sides. I replaced my trim when I refitted my hightop as it was brittle and cracking too.
with this....
...although this is not an exact replacement as it's a brighter white and
MUCH softer it was the closest I could find. Ok on straight bits or gentle bends but doesn't want to know about tight corners like on the rear of the hightop (wants to return to it's former shape) so I glued the odd section. It didn't occur to me at the time but perhaps a gentle application from a heat gun might persuade it that the new shape is 'home' once it's cooled down.
Anyway, this is where I sourced it but you might find a better solution out there somewhere....
http://www.ebay.co.uk/itm/Flexible-Edge ... 3a708e08cb" onclick="window.open(this.href);return false;