Ooops.. forgot to answer your first question regarding the amp under the rear seat.... I've never tried that in my CRX, but I had an 88 Civic before this car, and the only thing I ever tried to fit under any of the seats was an Audio Control EQL.

Initially, it was mounted under the front seat.... just barely fit after reshaping the lower seat pan slightly for more clearance. In my CRX, I think there is a little less clearance under the front seats..... but might be possible. An amp would be another story (unless it is one of those low profile amps... like the older Pioneer 4 channel ones, forgot the model, but the foot print was similar, but smaller than, an EQL and the heat sink was slightly convex, with the tallest point in the center being about the height of an EQL, maybe slightly less).

I also mounted the same EQL under the rear seat.... Like the CRX, the seat foam sits directly on the car chassis.... so I trimmed out a rectangular piece of the foam to match the shape of the audio control.... a panel on top of the audio control would protect it. That worked... but an amp might overheat since there is really no ventilation in that type of install. But now that I think of it, I believe the lower rear seat foam on the CRX is much thinner on the CRX, so this might not work out anyway.

I may have a few more photos I can dig up on the original amp rack/rear seat construction from my IASCA 2000 log book (it was originally built for three RF 200a4 amps... only recently, I swappd in the Xtant and made a new trim panel). If I find any.... I put them on another page on my site and post a link here.