GM uses a security feature called "theftlock" on many of it's audio systems. If the theftlock feature has been enabled on a GM radio, when it loses power it becomes inoperable ("locked") untill a previously programmed in secret code number is XXXXX If the code number is XXXXX the manufacturer has a second code that is programmed in at the factory that will unlock the radio. To obtain this code, you either have to beg or buy it from a GM dealer, or purchase the information from a source such as www.radiocodes.com . If the theftlock feature has been activated, the display will light but will read "lock". Since you state that the display is dead, I assume this is not what you are currently experiencing (but may run into once the radio is enabled to turn on).
If a late model GM radio is removed and installed in another vehicle, it must be programmed to the vehicle using a Vetronix tech-2 scan tool before it will turn on. THe radio has the original vehicle's VIN number programmed into it's memory, and when turned on it compares this to the VIN stored in the vehicle's powertrain control module. If they do not match radio will not turn on. The purpose of this feature is to prevent a thief from stealing your radio and installing it in another car; an inoperable radio is not a very attractive target for theft. All GM dealers and many independent repair shops have a Tech-2 scanner that can enable your radio to operate again; without one there is no way to turn it on again. If the radio's electrical harness in the vehicle is modified or has other aftermarket components connected to it, the radio will not appear on the data bus and will not be able to be programmed with the Tech-2 untill the vehicle wiring is returned to it's original configuration. This may be what your dealer is referring to when he is talking about aftermarket wiring; the system must be in as-manufactured condition for all of the electronics in the vehicle, including the radio, to communicate with each other on the vehicle's data network.
To make your radio work again, any modifications to the vehicle's original wiring may need to be removed to restore data communication between all of the modules. If it still does not operate, you may need to take it to someone with a tech-2 to reprogram the radio to the vehicle.
I hope this is helpful; if so, an accept would be most appreciated! thanks!