Ford Focus 06 Door Lock Problem

I have a 2006 Ford Focus. I installed a new car stereo two months ago. Afterwards, I tried to open the car using my key as I usually do and it would not open. I have the standard edition and there is only one locking door to enter and exit the car. Thus, I was locked out of the car. I figured out that I could climb through my trunk and was able to unlock the car that way. I tinkered with it for awhile and I turned off the car and disconnected the negative from the battery terminal. I turned the ignition switch twice and then put the negative back on the battery again. I turned the car on and locked the door. I was able to unlock and lock the door for two months. Today the same problem started again. I tried the same solution that had worked last time twice and it did not work either time. What is wrong with my car and lock? How do I fix it?

when lock with key or remote and then press unlock with remote or key. the car door will not open up used the handle. I will have to used the key and open the rear hatch back and go to front door door latch and open the doors

lion4500 Thanks for the reply. After months of dealing with this issue I am proud to say it is resolved! The ground wire has to be grounded to the metal around the stereo, not through the car. Once I moved this grounding, the problem was fixed!