Audio Systems: Aftermarket radio and stock amp??, kenwood dpx302, aftermarket radio

Question
i own a 05 impala base model and i just changed out my original cd deck (with out on star) and i had a store put in a kenwood dpx302 cd deck and it doesn't sound like my built in amp for my car is working why is that?

Answer
because that vehicle needs 3 modules to work
factory amplifier turn on module
on star radio module
speaker for onstar module only if you want to use the service
and data buss module

soundgate,pac or metra make this 3 parts

you need them all
good luck