As Basti said, the circuit is looking at resistance, and you have half as much as you need. An inline resistor should fix the problem, but I don't know off the top of my head how much you'll need.
At a guess, i'd say you could measure the resistance of one of your bulbs (multimeter on positive and negative of globe, which you take out to test), and then buy two resistors of that value and then wire them inline somewhere. (one for left, one for right)