I installed this in my Gt https://www.alpine-usa.com/product/ford-mustang-sound-system-psu-300mtg.
It was a noticeable improvement. I'm sure you can do better for $700 but it was plug and play, very to self install.
You might have used the wrong bracket for the wrong side. When I installed mine, the that "L" should be facing the center of the car. But non the less you got it to work. It probably isnt that big of a deal.
Oh, I did the same thing you did. Coated my car with cquarts uk a few years ago. Its still holding up might polish it off and try adams coating next. In hindsight I wish I ppfed the whole car. At the time of buying I didn't think id keep this car this long.
I would say get the all season 4 or look at the Goodyear Eagle Exhilarate. I would tell you to get winter tires since you are in Michigan, but if you can avoid driving on heavy snowfall the all seasons will get you through the year. Take a look at the cross climate two in the 235/50/18 size...