Of course it is, In the truck is much more common then under the seat actually... As long as you dont put stuff on top of it to cut off air flow to it so that it gets hot... Mounted to the back seat in the truck is a good place a lot of times, especially if they fold down then you can tune it or whatever else easily from inside...
Welcome to the forum blue - yay socal //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
It's common to mount them to the back of fold-down seats and I know there's a company that makes amp racks for subarus that attach to the roof of the trunk but give the amp room for cooling to preserve storage space; so if you wanted to you could probly fab up something similar. other people put them beneath the trunk's false floor... Just make sure it's not going to get too hot wherever you put it //content.invisioncic.com/y282845/emoticons/thumbsup.gif.3287b36ca96645a13a43aff531f37f02.gif