Ah, ok, the GMDC and GTTC are specific cables at PC-Mobile. They connect the Magellan to an Seidio cradle, eliminate all other cables. The Seidio then provides power to the Axim, receives GPS from the Magellan and routes it to the bottom port on the Axim. Audio comes out the Axim earphone connector to the mount, is amplified and presented to you with better volume and bass response. This solution meets all your needs except for power to the GPS. The Seidio cradle provides 5.4v for that, but the Magellan only wants 3.96 v (I think, check the specs). So, the Magellan would run on batteries unless you wanted to cobble together a power supply at about 4V and splice it into the PC-Mobile cable set. You might be able to use a power Y connector to run two power cables and splice your existing power cable for the Magellan to the PC-Mobile set.
Is
this the cable you have for the Magellan? If so, you could use the PAXGS cable on
this page. and that cable and get a simple mount and power supply for the Axim and all you'd be missing is the amplified sound. The sound in the X5 is pretty poor, though.
Or, you could use your cable, assuming it's that one, with the DIY cable from
Seidio, or get them to make a DB9 to RJ connector for you. I think they'll do that, I seem to recall them having that before.