I have experienced very weak reception with GPS receivers when they are near the screen or the power supply/CPU. Also, if the GPS receiver is not placed on an appropriate ground plane - a sheet of metal, you can also have weaker reception.
The BU-353 is probably the most popular GPS unit for car PC use. I doubt it is the receiver. Try placing the BU-353 on top of the car's roof with a clear view of the sky and as far away from electrical interference as possible and see if this clears it up.