The ipaq does have a headphone output, which when used causes the small speaker in the 5 way switch to mute.

Are headphone levels too high for the Mic input on the empeg?

Personally, I like Tony's idea of the small mixer board, but I didn't find any this afternoon in a 30 minute Google fest. The reason I like the mixing is because the 'spoken' instructions are quite clear when mixed into the music, assuming you have the tunes at a sane level. Occasionally I'll use WMP on the Ipaq for really new tunes not yet on the empeg. I use the headphone out to the Aux in on the empeg. When listening this way the GPS 'voice' is nicely mixed with the music over my speakers.

The small board seems like the simplest way to go. In my case I'd insert it after the Sony head unit switch, so it'd work with either source. The design of such a thing is outside my ken. (I'm a steel, plastic & aluminum kind of guy)
-Zeke