If you put the GPS under the steel roof, it can't see the satellites, because reception is lin-of-sight, and all you have is a fancy clock. You 'll either need an external antenna or dash mount it.

I am now the proud owner of not one, but two Magellan 4000XL power/data/antenna modules, one of which I've modified to be power/data only, because otherwise the internal antenna is disconnected and you can only use the GPS with an external antenna, which I don't own. I now no longer am responsible for Energizer's entire bottom line, and the bunny will starve. These things are interesting. You can pull GPS power off a computer if you use a voltage doubler chip gizmo to take the computer's serial port 5v to 10v, or you can split the unterminated cable into the wires for the serial connector and the wires for auto (or boat) 12v.

For my money, the best place for a gps in the Monty is between the main instrument console and the center pod.