Once these critters get into an attic they can be quite difficult to remove. Even more so in winter months when they start looking for a place to bed down for winter.
You need to do a few things. You need to try to eliminate their access to the roof or where ever they are coming in. This may require cutting branches away from home. which is important in many other ways as well. Also any bird feeders or food sources need to be controlled to keep them away.
You need to seal off the opening once you have chased them out. Metal works best, but some folks have had success with steel wool, Applying grease on the surfaces that they chew on helps to some extent as well.
Trapping is sometimes required. There are several live traps you can rent or purchase that will do the trick. Once caught, you need to take a very long drive or they will come back.
Once you know you have removed them, you need to check your attic carefully for dead squirrels and all the wiring that may have been chewed upon. For what ever reason they like to chew on the plastic and paper wrappings.
Ideally a professional pest contractor should be consulted if these methods do not do the trick. Once established these critters can be quite hard to remove.