Window Tint's effect on GPS signal
It would appear that the degradation of GPS signal into my car was not caused by the defroster lines, but instead caused by elements within the factory tint on the rear window.
For many weeks I was experiencing low GPS signal stength and temporary losses of satellite lock. About ten days ago I replaced my factory tint (35%) with a new darker limo tint (5%). After testing the GPS with the new tint I found that performance of the GPS receiver had increased significantly. The GPS can now lock quickly (5-15 seconds) even while initializing at high speeds.
I'm assuming the factory tint was composed of a more metallic film than that of the aftermarket tint.
I hope this information is useful.
Vehicle: 97 Lexus SC300
GPS receiver: Delorme Earthmate USB w/ latest drivers
GPS software: Routis 2004
GPS reciever location: passenger side of rear deck