headlights

Check for voltage at the plug with a volt meter. If the high beam works then its most likely not the ground. There is a possibility its the ground wire itself but not as likely as a power wire.

high beam is separate from low beam .When checking for voltage meter goes to power then no power at all

What I was getting at is the high beam and low beam ground wires meet at one ground so if the high beam works then its not an issue at the ground itself but may be a wire issue. When you turn on the low beams you say the plug gets voltage then it loses voltage right away? All voltage or does it just drop low? If it loses voltage slowly then its most likely a high resistance in the power wire between the relay and plug. Test resistance from the relay to the plug.

What number bulb is the high beam on a 2000 gmc serria 1500 ext cab