i just made a new rear door card (faaaar back one, the one with the access hole to the rear washer container) and fabbed up some quick brackets to the supports behind the card. all you can see when you look at the card is some power outlets, the rest of the inverter is conceiled. along with a couple cig lighters i wired up to that card, along with a recessed old army first aid kit, it's pretty cool looking. gets plenty of air back there too, its not snugged up against anything really.
i'd take some pictures were my camera not stolen recently. i plan on covering the new door card soon anyways and was going to snap some pics with a borrowed camera, i'll post up when done.
there's quite a few places you could hide one though. i'm not the biggest fan of underseat mountings, be it an inverter, amp, cd changer, you name it. just gets to dirty down there for my likings. and i have had water get up to that level before too, be it from a river crossing or leaking windshields/sunroofs from the rain. mounting them up a bit is nice security. these cars are big with lots of hiding spots, plenty of options so don't shy away from getting creative.