Codes 88 actually means a few things
88
Throttle Kicker Solenoid
Variable Voltage Choke relay circuit fault
Fan Control circuit fault
A4LD - Converter Clutch Override solenoid
Electronic Ignition - IDM, DPI or spout circuit fault
That's 5 things for one code. And which ones depends on what car. Mustangs don't have throttle kicker solenoids. At least I wouldn't consider the IAC one.
It's definitely not the middle 3 as you don't have a choke. You dont have an electric fan and your car doesnt have an A4LD.
I think yours is the last one.
Electronic Ignition - IDM, DPI or spout circuit fault
With that said...is your SPOUT unplugged??