Try doing one arm (or both) reverse
grip pulldowns on the
cable machine. Stand in front (say you are using both arms) and with a straight bar attached, instead of using an overhand pushdown grip, reverse it to look like a bicep grip. Now, pull down the weight with those triceps and at the bottom, you should feel it in that rear head.
EDIT: Perhaps now that I think about it, you mean the "outer" head and not the rear like I was thinking. Do you do Skullcruchers or overhead presses at all? Try some of those too.
