o2s

I have a 99 gmc sierra 5.3L z71 ext cab 3 doors push button 4x4....check engine light really really bright lol..I read to codes and it is o2 sensor and heater 4 codes 2 for 02 sensor both sides and heater both sides....My question is does anyone know where the ground for the o2 sensors are found on my pickem up truck??