well you have 2 options here. i would be tempted to join into a constant live in the boot if there is one(stilo boys will know)since were not talking big power here. this would save runnig a wire the length of the car sine being a newer car with nice trim it can be a bit of work.
all you would need to do there is splice into the constant live with scotchloks
and then again splice into the light live.
if you want to do it properly then you would need to connect a wire to the battery live. a ring connector
over the 10mm bolt would be easiest with an inline fuse.
and run this wire through to the boot(theres a guide in the stilo faq's on running power wires through)
then do as i said in the first part,running the live to the live of the lighter socket.
now if you doing this and want to use the solar charger a fair bit you could actually leave the lighter socket alone and hard wire the charger in. the charger only has a + and - or + and earth as i'll describe it. either cut of the charger lighter plug or to keep both options connect the positive to the charger via scotchlok. now for an earth you need to find a bolt on the bodywork.usually a few in the boot(stilo boys will say where)and again a ring connector on a wire under the bolt is the easiest.
this is connected to the other wire on the charger cable.
think ive made this sound more complicated than it is