I have a 78 EC. Everytime I hit the brakes or turn on the wipers there is a pop in the speakers. I put in a digital style factory radio from a 87 Monte. I have grounds all over the place and just replaced distributor. What would be causing this ?

Where are you picking up the radio 12 volts from? The original yellow radio wire? You might try a filter choke in the 12 volt line. Available from Radio Shack. Not sure it will help. Radio have a good ground to the frame?
Sorry, the only external ideas I have. Problem may be internal to the radio.

I am using the factory harness, it plugged right into the newer radio. I also have one from a 1994 Lumina that I would like to put in. The plugs are different but Circuit City has adapters, mounts look real close.