yardost

Have a 1999 GT convetible and occasionally run into the following problems:

At times, after the using the passenger door, the interior light would not go off. You can open and slam the passenger door as many times as you like, the light would not budge. Only option is to let the car try and turn it off in either 3 or 15 min auto timed interval which only works some of the time. Other times, you have to just drive around for days with the interior light on, even if you are not using the passenger door. Eventually you would lucky and the light would go off.
Also, at random, passenger or driver side quarter window would refuse to go down. The next time you take the car out, the windows would work as if there was never an issue. Not able to narrow down any specifics on the above as they would happen at totally random times.
In addition, there is a whining noise from the speakers when stereo is on.

I have ordered a replacement door latch but I am thinking that might be a wasted effort as today the driver quarter window refused to work and it reminded me that the new latch would not address that issue. I am wondering if there is a ground issue and what would be the best way to troble shoot it? I have tried taking it to shops but they want $85 per hour to trouble shoot with no guarantee on how long it would take them.

Thanks in advance.