did you fit a fog light relay? if not, this is why the voltage is low. see the fusebox faq for location, it needs to be a #53 relay like for dual horns, windows etc.
also, even when fully powered up they wont give out much light especially over main beams. they're designed to light up the road underneath the fog, not provide a shat load of extra light. if you want extra light at night use the main beams, thats what they are for.