You need an LED Load Equalizer. I have LED's all the way around on my Denali and SY. You need one for each side because LED bulbs do not carry a high enough load so the vehicle thinks there is a bulb out, hence, the rapid flash. Follow the link and move down. On the right you will see the 3 ohm Load Equalizer. You will need 2, (right and left sides). http://autolumination.com/equalizers.htm
-Deano
Ther is an easier way... I'll have to take check again but I think you just add a resistor the ground of each bulb. I did it on the GT in the parking lot at NATS '05