Ok, so I got the PAX FRD04 aux audio input for my Shaker 500 in my 2006 Mustang.

I've got the dash all apart no problem.

I've Got the radio out, hooked up the interface between the radio and factory connector.

I've got a power source to the PAC from a "on with the key" fuse from the passenger side.

Put everything back together. Turned on radio, all speakers work. (Hurray!)
Hit Aux input on the Shaker 500.

Shaker 500 tells me "no aux input"

What did I do wrong? Do I need a better ground?

 

well do u have a good ground?

 

unlpug the FACTORY radio plug from the harness(leaving the AUX input plugged into the radio) and let the radio sit for about 5 minutes to reset.Reconnect the factory radio plug,turn the radio on,push the AUX button and see if it works.

 

Unplug the negative terminal of the car battery , that way there is no need to take the radio apart again

the first time the radio gets power, after is been unplug for 10 minutes it search for devices plug into the radio, most likely this will solve the problem

 

After relocating the ground, I did this. The PAC worked. Hurray!

The next time I started the car, it didn't work. So I took the dash apart and unplugged the harness and plugged it back in again. ... it worked.

The next time I started the car, it didn't work. So, today I took the dash apart again, and unplugged the harness.... and returned it.

Maybe I can find a Shaker 500 from a 2008 stang with the aux input built in... or maybe a Double Din Nav Receiver.

 

I went through the same thing you did...I found the PIE adapter to be of very poor quality, and it just would not work right. So I returned it and bought a Pioneer double-din.

 

I just got the PAC adapter for my stang, I wanted to do the headunit swap, but don't have the money for it right now. As for your problem, you have to hook the unit to a constant 12v source such as a the cigarette lighter on the dash. The problem you're having occurs because the adapter has to be on before the radio turns on. Hooking the unit up to a switched 12v source turns the adapter in the same time as the radio, and produces a communication error between the unit and receiver. Hope this solves your problem!