Hi Johnnyairtime,
Yes, 'Al' is quite correct, it is unlikely to be just a resistor.
and he is also correct that the various voltage stabilising chips
would do it quite well.
However on motor-bikes the norm is to go for the simplest and
most basic method, and for this the simplest is a resistor and a
zener diode.
Yes, it is not power efficient, but my guess is that the GPS unit
is a pretty light load anyway.
I have included a diagram of the sort of thing i would expect to
find doing this job. When you get into it, if it looks like this
is not the arrangement, then please describe what you find, and
someone, or me, or Al, will hopefully know what it is.
I expect you to find a loose or poor connection, with any luck
you will not need to be concerned with the unit, other than that.
Best of luck with it, John