Ideally, unscrew the connector from the back of the GPS, unwind 3 miles of spiral wrap, pull the cable up through the A frame and out, then re-run the cable with the new base in situ.
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or, you could cut the cable and reconnect using a soldered joint and heatshrink, but that would invalidate the warranty of the antenna, if it still has one. I think the GA29 uses coaxial cable