Using the automotive GPS simply to get co-ordinates, then feeding that data to a laptop
with charting software, should be relatively straightforward. I think this is what Dave is doing. There are some car and phone
GPS units that are software-locked in some way, or simply don't have the physical interface, and so can't provide useful data to the user (only to the embedded firmware).
I doubt you'll have much luck trying to create and install new firmware for an automotive GPS. It's theoretically possible, but probably not practical (the time and effort spent reverse-engineering the car GPS, then reprogramming it, would be far more than the cost of a unit designed for marine
use). Since you wouldn't have any prior knowledge of the thing's hard-wiring or source code, you'd pretty much have to start from scratch and would likely brick dozens (or hundreds) of the things while experimenting.