Nice install.
I have a Montego(Mercury's version of the Five Hundred) and I feel your pain when it comes to installing radios.
In my case I didn't have any remote head radios and I didn't want to buy one just for the purpose so I wound up mounting them inside the center console, under the armrest. That location is both a benefit and a disadvantage. It's a disadvantage because I have to open the console to access my radios(2m and CB) but it's an advantage because my gear is out of sight from anyone that might want to swipe it. I mounted the speakers just barely under the driver's seat, so that they are sticking out an inch or two. I can hear them pretty well that way.