I had an underseat sub in my MR2. Installation is pretty easy, the hardest part is getting power from the battery. The power cable supplied with mine was a few meters long so you shouldn't need to worry about that. You don't need to cut any speaker wires if you use an RCA cable (the cable with a red and a white coloured ends, also known as a phono cable). Make sure you buy a shielded RCA cable though as otherwise it will pick up interference and it makes your subwoofer sound like a turbo when you accelerate!
If I decide to install mine then I'll write a 'How-to'.