hey, I have a ton of equiptment on sliding drawers under my 2 rear seats!!! I even built new seat mounts to accomidate these... the brain of my iva-d300 is under there, with the brain of my h700... the d300 expansion module, a 4 into one unilink box & the video box for my 15.4" overhead dropdown!!!!
I ALSO have a bunch of equiptment under the front passenger seat including a netgear dual hardrive network enclosure and a kyocera kr1 wireless router!, I'm adding another motherboard under there soon probably as well...
If I couldn't put equiptment under my seats I'd have no room for passengers
ah... maybe I should mention that my car is a minivan & the seats are a bit higher than an average car... PLUS I have stripped my interior to the bare & everything is prepped to accept this setup properly.... I even built a center wire tunnel that runns the center of the truck for the many wire harnesses that I have going on..... I can run a wire at any time from the front center console to the rear of the truck with a snake through a tunnel
the end story is, although everything you guys/gal has said has merit, there can be exceptions to the rules
red has pointed out how grounding points can be problematic under seats, & while this can be true, & a seat bolt can usually be less than ideal, this doesn't mean you can't run a ground TO under the seat along with whatever power your running...., & have it come from a good grounding point..
I'd also say that the biggest problem I see with under seat installs is that there not done right most of the time.... but if done correctly they can provide valuable space
you have to realize how much the center of the seat can come down when ocupied(a lot!), & you have to make sure you have more than adiquate clearance for FULL sliding back & forth, & up & down if applicable.... I have had situations that I have utilized this space & have had to limit the travel of the seat mechanically to accomidate it.... NEVER just trust that no one will move it!!!
most of the time to do this properly will involve removing the seat, & possibally building a mounting board, or pedistal brackets..... under the seat is NOT where you just want to slide something under there..... laying on the carpet.... that's what gives underseats a bad rep, is because most of the times it's done sloppily & you have to also account for bad weather conditions..... you'd be surprised how trashed the underseats can get in snow & stuff... if you use a bit of forthought the seats can be a good place for equiptment, if done properly...
also keep in mind that equiptment that requires good airflow can be problematic, amps can be problematic.... under seats is usually not an ideal space for equiptment, but not ruled out either......