A great Idea that I haven't gotten around to, yet. Since you got me thinking how I would do it I will share my thoughts.
What size inverter? I want to be able to run power tools but not from my dash so I'll put the monster inverter in the back. For the dash, a small one around 400 watts will work fine for laptops and ni-cad chargers. Make sure it's rated continuous duty for what you want to run in it, not PEAK or SURGE.
Example: Your laptop wants 300 watts and maybe you want to charge some camera batteries (55 watts) at the same time. You would use a 400 watt RMS / 800 Peak inverter. These are small, quiet and pretty cheap. Go pick one out at the store.
Hook that baby up normally and plug in your max load like the laptop with low battery or whatever will push the inverter to it's rated RMS limit and let it run a bit to see how hot it gets. Run it inside a closed shoebox. If it stays cool then you are good to go as far as tucking it away under the dash or console. If it gets pretty warm I'd leave it outside the cab or keep it ventilated. Also, this stresses the unit to see if it will run well before the store's return policy runs out.
Now that you have the unit and mounting location, you need to feed it. DC has issues with voltage drop over long wires so use beefy wires if going to the battery. 14ga wire (A piece of old outdoor extension cord) will work fine with a 10 amp inline fuse and switch (a lighted one would look cool and geeky). If you put it in the console, attach some stand-offs to the inverter so it don't sit on the carpet for better airflow around the unit.
It's time to put in an outlet. I will use a panel mount receptacle or two. You can scavenge them from an old power strip or get them from someplace like
Digikey. Number 17 looks spiffy.
The receptacle will run fine with 18ga wire using a small inverter. I will use an old vacuum cleaner or small appliance cord (with ground) that is comfortably long enough to reach the the new outlet and it already has a nice molded plug on it. For a 400 watter, a 5 amp inline fuse here will give you more peace of mind.
The rest is location and aesthetics. My only concern is that the electronics in a vehicle may not be well shielded against the EMI from an alternating current source under the dash. Maybe a shielded 3 conductor cable from the inverter to the outlet should be considered.
Anyway, have fun with the project and resist the temptation to run a spare set of outlet wires to the seat your mother-in-law normally occupies on those long family trips.
Let us know what you do and take some pics of your handiwork for others to drool over.
