the code i receved told me bad knock sensor.
can anyone tell me where i might find this on my 99 gmc?

Remove the intake manifold. Underneath you'll find the knock sensor wiring harness, remove that. Remove the sensors. There's a TSB with regard to the GMC knock sensors whereas water will get on the rear sensor, and cause it to rust. You will see a gap at the rear of the manifold, where you will have to build a Dam with silicone to prevent that from happening. Good Luck