I hav a 2004 Chevy Colorado 4x4 ext cab 5 cylinder. I put in an aftermarket stereo and viper alarm no problems.. then put in an amp. Now it won't turn over. The lights come on but when u turn key dead

So , what did you do about the class 2 serial data bus ? Do you know what that is ? And it is suppose to be hooked up too the radio . With aftermarket stereo's on GM vehicle you need to use a interface module . ! Have it check for DTC'S - diagnostic trouble codes , you will probably find a U - code , communication network failure .

Each module connected to the class 2 serial data circuit monitors serial data communication during normal vehicle operation. Operating parameters and other information are exchanged among the modules. When a module receives a message that contains a critical operating parameter, the module records the identification number of the module which sent the message. If a module does not associate an identification number with every critical parameter within 5 seconds of beginning serial data communication, DTC U1000 or U1255 will set.

Something on the bus is not letting normal communications happen .