The power plug that is under the seat on the GV250 is switched by the ignition switch.
So it only connects to the battery when the ignition is on.... not what you want for charging your battery.
If you buy a name-brand battery maintainer (Sears Craftsman or Walmart has Schumacher brand for $20),
it will come with a short wire that connects to the battery posts and has a 2-wire connector on the other end.
That end connects to the mating connector on the wire from the battery maintainer. So all you have to do
is to connect the short wire (pig tail) to the battery permanently and then route the other end to where you can get to it.
We leave the connector end under the seat and route the wire from the maintainer up by the rear shock into the
under-seat area when we want to charge the battery:
Open the seat, reach down by the shock with 1 hand and up holding the wire with the other hand, pull up the wire,
connect the wires, plug in the maintainer, & close the seat.
I am sure that you could use a longer pig-tail and secure it outside of the under-seat box,
but it wasn't worth the effort to us since we ride most of the year.
Scoot safely,