http://www.brainboxes.com/
Might be able to provide the basis for a solution but I think the problem that your will need to overcome will be supplying power to the transmitter as I doubt the RS-232 on the GPS si configured to supply power. This could well require hacking a serial to BT unit to add external power.
How about the fast and cowardly route. Pop the old GPS onto ebay and buy a new one
