antenna/reception problems

Most of your problems stem from you using the incorrect harness. The vehicle you have has a data bus system in it that is called class 2. In a class 2 vehicle the radio is on the data bus and is controlled by the BCM. That is why you only have a constant wire because the BCM turns the radio on digitally. Now by using the proper harness with a computer chip in it to replicate what the factory was doing would solve all your problems but the reception. As far as the reception goes understand one thing, nobody makes a better am radio than Delco. Most any old time installer or Delco tech will tell you this. So anything you do besides the factory radio will be of lesser quality than you had. Also, anytime you have break in the signal chain (antenna adapter) it will cause more resistance. AM relies heavily on a good ground. Also to make matters worse is that most aftermarket radio manufactuers do not concentrate too much R&D to AM. I have dealt with this quite often. So, in your situation do this. Get the proper harness, which I use a Metra piece (GMRC-02) and make sure the radio you are using is known for having good reception. I have used Pioneer and Alpine with good luck. Also, I don't know if they still do or not but Delco use to make the tuners for the Rockford Fosgate radios.