If it's a single extra lamp you're after it should be possible to 'add' it to the existing wiring without overloading it, but really it should be done with a second (or uprated) relay.
I have fitted bulbs of (I think) 100/130w without a changing the relay and without frying the cable. However, although I haven't actually done what you intend, I would splice into the cable at or close to the switch. If your AT is one of the continental versions that have only one headlamp lit, and you have already made the modification to operate both at the same time, then you would already have a second (or higher rated) relay.
Hopefully someone will read this who is a qualified auto electrician (my modifications usually work, but more from luck than judgement!). In the meantime it might be of some interest to look at http://www.xrv.org.uk/articles/headl...onversion.php.
Good luck! Stig.
Tall men see everyone else's bald patch but their own.......