Be careful there, Keith. Halogens draw significantly more current through wiring designed for lesser lamps. I burned up a headlight switch on my old coach doing just what you did.
As Gil mentioned, a relay is best, and most are so constructed. But you may need to replace the relay-to-lamp wires, relay-to-fuse wires, and perhaps more to get the proper sized wires and fuses to handle halogens. You could be driving along one night and be suddenly plunged into darkness as a component fries on you.
Seems there was discussion on this in the past where someone supplied a link to a fellow's website where he sells kits for resolving the issue, and many commented that it did the job. Can anyone on the Forum supply the link again for Tom?
-Joel