Need replace because of S68/3/, S68/4

How difficult are they to replace?

Just replaced the mass airflow sensor, it's now starting, but rpm quickly drops and dies within 1 2 seconds, if i rev the gas i am able to keep the rpm up and keep it going, as soon as i stop it quickly drops and dies,won't stay started more then 2 seconds without keeping my foot on the gas.what is causing this problem and how do I fix it

When I stop! I have to put it in park then back in drive to go.

when i first start it cold , it will start with little effort, but as i start & stop it doesn't want to start unless i give a little gas & it will start then, i checked fuses , or could this be fuel pump getting weak

When any of the doors are open and not locked it won't do this. But when all the doors are closed snug and locked and you hit the trunk release button on the console it unlocks the doors and pops the trunk. I've had this car 8 years and it never did that before. It would only open the trunk. I had major work done on the car and when I got it back it did this. The shop is telling me it was supposed to do that all along? I need to know what it's supposed to do and the manual doesn't specify so I can make them fix it if they took a shortcut that's now making it unlock the doors too.

I have a small coolant leak coming from the back of the head.

The belt took out vacuum lines and wiring to air switching vacuum valve. Destroyed the connector and wiring to this valve. I need to know how to rewire or to get a new plug/connector. I can't afford to replace the entire wiring harness. Thanks in advance for any help offered.

MY AUTO STARTS EASY AND RUNS ON ALL 6 cylinders, THEN sounds like its running on three! Then quits after about 8 seconds.