Eeek... I'm not an expert on this... but I do know that if you have 300 watts...that's pulling about 25 amps on your cig lighter...I would probably not do that; I would run some heavier gauge wire from another power source. And if it's not already fused, I would fuse it. And I'm not sure...but I think on some cars, the cigarette lighter will remain powered even with the ignition switch off.
For question #2, that would depend on your vehicle. Mine is probably going in the trunk somewhere. But if you have a pickup truck of course that won't work. Another place, if the computer case is small enough, would be under a seat (again, if there's room). Just make sure there's good air circulation.
Hope this helps
As nitro said, run a heavier gauge wire from the batt positive terminal, fuse it no more than 30 cm away from the batt terminal and to where your inverter is. Make the ground wire as short as possible because you are bound to have interference problem if not grounded properly (especially with the inverter case).
AWG recommendation for positive (from batt terminal to say boot approx 16ft) 10 - 12 AWG should be fine. For ground i would make it about 30 cm or less with 10 AWG.
You should add a relay to the wire as it will be always on for the inverter input.
Hope this helps..http://www.the12volt.com/info/recwirsz.aspv
cheers guys for the help.
One last question what GPS software would you recommend and where could i *download* a copy from lol-