GPS installations is one of the things I did for years. The issue you are addressing is so installation specific that without knowing a lot more about your specific case, there is not much to be done.
Sometimes, one of the stand-alones in the cab would track for most of a day if we were lucky. A friend borrowed the Garmin and it had no problem on the dash of his C.
So, yes, it can be a problem. My advice, try it in different places on the dash. Look for the place that the GPS can see the most sky.
I used to stick the receiver for my laptop on the hood of our Cayo/Dodge, but with all that metal, there was nothing that worked in the cab.
Matt