So I just got off the phone with Trump...
In his write-up about his speaker installation, I noticed he has all the positive/negative wires for the inputs all coming together at the speaker.
This is sure to cause some problems as speaker output from the radio, gps, etc, is UNI-directional, it may also be causing the interference he's getting with the GPS.
All of this can be solved with a simple mixer, which makes sure there is no power from one unit traveling into another via the speaker wires.
The one I've used in the past is from Azden...
http://www.azdencorp.com/new/product.php?productid=93302&cat=3&page=1
Azden's retail listing is a bit high, but you can find it much cheaper on ebay, here:
http://shop.ebay.com/i.html?_trkpar...&_sticky=1&_trksid=p3286.c0.m14&_sop=15&_sc=1
OR... If you're an electronics buff, I think a diode of some kind will prevent the voltage from traveling back up-stream.