Alpine Reception problems

can anyone help. I put in an aftermarket Navigation system 2 years
ago. this is my third fall/winter with it. it seems as if each winter
it stop receiving most satellites . now it seems even worse. i can
drive 20 minutes until it picks up my location and then only finds 4
of 8 satellite... my other cars has a built in unit and it works great
right from my drivewawy. can the seasons have an impact on this? any
clues? i live in North NJ?