I have just changed one in my 2004 Tahoe because I had the same problem.
there are only 2 screws but very hard to get to the top one. I used a 7/32 wrench and took my time.
but what is important is that you make sure the cam plate behing the accuator stays in place when you pull off the acuator.
when placing the new acuator on the cam plate if it does not fit on the sloted shaft plug in the wire and turn the key on the actuator should move into place.
If that is not enough while turn the cam plate behind the acuator making sure the plate stays in place.
If the plate comes off there is a pin on and a plastic gear you will have to set back into place and with the small amount of space it is very hard set up.
I am sorry I am not providing detailed instructions but it is hard to explain but once you set up under the dash and you see there it is what I am saying should help you.
once you have all this in place you have to pull the fuse under the hood while the key is set to the one position for about 10 seconds then replace it and start the motor. this will calucalte the acuator.
One last thing make sure you purchase the correct acuator there are two that are listed and the one to use of for the dual unit about 150.00 is what I paid for it.