my car wont turn over. on 2002 GMC Yukon

we've checked the battery and changed the cable heads

Asked by for the 2002 GMC Yukon
Do the head lights come on and shine brightly? If so the battery is probably OK. Do the dash warning light come on when you turn the key to the ignition on position, if so the ignition switch is likely to be getting power and transferring power. Ignition key in the start position check for power at the small wire at the starter, it will only have power when the ignition key is turned to the cranking position, if it has power under this condition it is likely a bad starter/starter solenoid.
There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate
yes ck your lights. there is a bulk loom of grounds at that rear of the p/s intake by the dist. that could be loose or corroded ck those