alright so it looks like Metrompg.com already did this for us, but the results there were about 230 total watts to run the stock systems.
So a system to compensate the alternator would need 2 of these(min) plus a deep cycle battery.
You could run most of the draws off of the panels and then the rest from the battery and if your commute is not too far you might get away without breeching the "deep" end of the cycle(extending battery life somewhat).
If you go with 3 panels you are in the clear for most situations(including running cooling fans).
But this puts me out unless I mount them on my (totally) flat trunk. So the cost would be about 840 for the rigging and if you have the interior space no aero losses 40 lb addition or if you have a convenient non aero-influenced area.
So it would take me about 400 tanks to recover the cost. Thats 4 years. Alot better than conventional solar panels mounted on your house, but still a long ways out.
I'll have to work on some serious aero mods and weight deletes and lrr tires before I can get that under maybe 2 years.
<edit>
http://sunelec.com/index.php?main_page=index&cPath=30 has a 205 watt panel for 508 each but you have to buy a pallet of 28. So if you can find someone who wants some panels and get together and get a palet you can get it a little cheaper. Downside is the panel is 5 feet by I think 3.5 feet so its a trunk/roof only application. Unlike the metrompg.com page this does not have legacy costs(as long as you don't break the panels) because once its in they last 15-20 years. If you also go EV in the future its a bonus. This reduces the ROI to 2.5 years as my car stands now, lower cd higher mpgers would be able to get 1.5-ish </edit>