1999gmc yukon,the battery is new but if let sit 48hours the battery is dead,also the running lights and brake lights will not work,when pushing the power lock buttion the interior light comes on

Try the specified fuses in the under hood fuse center. they are common to blow.

Check the brake lights when this happens, if they are not on then you have found the problem it is the brake light circuit. Go under the dash if the brick lights don't work and check the brake light switch above the brake pedal, it should have 12 volts in and out with the brake pedal pushed down. See typical brake light switch location below. #4 in diagram is brk lite switch.

Sounds like you are on the right road with the switch. Before I bought a new switch I would locate the switch, it will come in direct contact with the brake pedal arm, and see if there is an adjustment or if it has moved for some odd reason. Sometimes things just happen.

Giving it a boost should work. Could be a bad battery or a short in the electrical system causing a 12v draw which will run the battery down.

First of all, disconnect the negative cable from the battery and charge the battery up to a full charge. When that is done, check the battery voltage with a volt meter; you should see about 12.5 to 13.5 volts DC. Next, reconnect the negative battery cable and make sure all cables and connections are clean and tight at the battery and the starter. Start the car and check the voltage at the battery again with the car running. You should see about 13 to 14.5 volts DC which confirms that the alternator is doing it's job.
Next, check to see if your brake lights are on when no one is pushing on the brake. If they are on, then that is what's draining your battery.
If that is not the problem and your car battery continues to go dead after a couple of days of sitting, you have some sort of "parasitic load". You will probably need an automotive electrical expert to find the parasitic load.
If you find that the brake lights are on when no one is pushing on the brake, you most likely have a little plastic piece broken that mounts near the top of the brake pedal shaft. I believe it is called a "brake pedal stopper" and it is supposed to push against the brake light switch when there is no pressure applied to the brake pedal. When it breaks, you should find the remnants on the driver's floor area. You can get a new one from the dealer for a couple of bucks. You may want to use a flashlight to check things out near the top of the brake pedal shaft to see where the brake light switch is supposed to be pushed by the "stopper".
I hope this helps.
Good luck, Mark

Remove bulb, make sure it sits tight in the socket. make sure socket is not burnt up (brown or black in color) Check bulb, if the filament inside is loose or discolored, replace as nesessary. Check brake light switch under driver side dash above brake pedal. make sure it is adjusted correctly.

From sitting, the brake shift interlock solenoid has probably become stuck. There is a manual release near the shifter--you merely insert a key to push down the release. That will get you moving. With the engine running, try gently and rapidly pumping the brake pedal to get the solenoid moving freely. Be sure the brake lights are working.
In the worst case, you'll need to open the console to free the mechanism or replace the interlock.

We had the same issue with our 2007 Yukon Denali XL. Died on us twice after sitting for short periods of time; once we watched a DVD with the engine off and battery died within 10 minutes. The dealership kept it overnight to troubleshoot why the battery died so quickly. They said the radio was "pulling" too much from the battery. (They supposably fixed it) But again a few days later the battery died again after sitting overnight. We brought it back to the dealership and they brought in a GMC "expert" to replace the entire DVD/CD/Navigation system. They also replaced our battery with a much larger battery, similar to the ones that GMC puts in there diesel trucks. They had our Denali for 5 days, but it appears to be fixed now.
Getting it fixed correctly wasn't without headaches, but it was all covered under warranty, including the battery. And they are updating the nav w/ the new edition disc as well for no charge.

new brake light switch,check that a mouse hasnt munched cables under bonnet or behind dash.Then tour dealers and recycle this beast and buy a nice low emission eco friendly car and do your bit to save the planet

Try to check your brake lever pedal by lifting it. you might have a bad brake light switch. If it solve the problem its time to replace it. 1. Just remove the 2 wires connected on it and remove the switch.2. Replace it with a new one and re install the wires ( plug and play socket) and your good to go. hope this would help you.