gpu upgrade on mac pro 1 1

I'm trying to upgrade my GPU on my Mac pro 1 1 to an R9 280. I do not get any image in the display. I've tried resetting the GPU, and I do have the 6 pin power connectors hooked up. Any ideas? Also, the gpu_present led is off, but it's like that on the stock GPU as well. This GPU works in other PCs fine.