It just sits at the "waiting for signal" screen forever.

Hang on a second.

What is the EXACT TEXT of that screen?

If it's something like "Waiting for data from satellite receiver", then it's a problem with the serial cable like we've been working on. But if gets past that screen up to the "Searching for satellites" screen (without you pressing any buttons), then it's communicating with the earthmate just fine and the problem lies elsewhere. So which is it?