need help installing rear fog

hey guys i have a '92 bmw 3 series and i need help wiring my rear foglight. i know what you're thinking, why would anyone want to have a rear foglight. well this is actually a safety feature, in bad weather (fog, blizzard, rainstorm) it will make your car more visible to the car behind you. anyway, my rear park lights consist of three bulbs on each side. one is a 21 watt single contact bulb for the brakes, another one is a 5 watt single contact for the park light and the last one is a 5/21 watt double contact bulb.. this is the one i'm planning to modify. only the 5 watt section is wired. so i got a wire and slpliced it to the positive wire going to the 5 watt section to an inline switch and then to the 21 watt section. when i turned on the park lights, the switch was lit up even if it was in the "off" position. the 5 watt lit up and when i threw the switch the 21 watt section didn't light up. what should i do? i tried using the positive wire from the other 5 watt bulb but it still didn't work. help!

BTW:  the current going to those lights...on tht beamer...isn't strong enough to turn on the foglight...Đễẳ†ђş 12\/ Ĵęśŧē®™