Hate to sound mean, but that's like asking, I pulled my old motor and put in a new motor and now it's not running what did I do wrong. It's a vauge question with no easy answer.
In short, you need to recheck your wiring. When you install an aftermarket unit you need to use an adapter harness, if you don't it won't work right. You also need to carefully follow the wiring diagram to ensure you're connecting the factory wires to the correct wires on the back of the new radio.
16 Mustang GT - 390 RWHP 370 TQ
13 Mustang V6 PP - 270 RWHP 260 TQ
95 Mustang SVO - 400 RWHP 400 TQ sold
13 Mustang GT CS - 380 RWHP 370 TQ
08 Mustang GT - 285 RWHP 305 TQ sold
99 Mustang Cobra - 280 RWHP 290 TQ sold
11 Mustang V6 - 270 RWHP 260 TQ sold
97 Mustang GT - 190 RWHP 250 TQ sold
00 Mustang V6 - 145 RWHP 184 TQ sold
98 Mustang V6 - 125 RWHP 210 TQ sold