Man, I feel you! That RTR insert is a game changer for the twin tip look without swapping the whole valence.
About the harness newer GTs might have pre-wired connections for the active exhaust, but it’s not always plug and play. Your idea of a 4 way switch to manually control the valves could...
A black rear wing on a Grabber Blue Mustang GT really sharpens the look, giving it a more aggressive, sporty feel. The aerodynamic benefits are small for street driving, but the visual impact alone makes it worth considering. Quality and fit matter a poorly made wing can look worse than none.