First off, do you have a manual or an auto trans? If you have an automatic, you can remove and drill a hole through the plate that's covering the hole where the clutch pedal would go through. Just be sure to use some sort of grommet afterwards to prevent the metal from scraping away at the insulation on the wire. That's the way a few people I know with PTs have done it.
Second, if you go ahead and remove one of the bolts holding the seat bracket to the floorboards, you can use a ring terminal that's either crimped onto or soldered to your ground wire, just place that on the threads of the bolt, replace the bolt in the bracket and tighten. That's how I have mine wired as well.
If you go this way, and you find that you have a small amount of whine or interference, it's because this particular method grounds the amp to the car's common ground, meaning you're grounding to the same point that all electrical devices in the car are grounded to, so the solution for this is to grab a couple ring terminals and some 12 (or so)-gauge wire and run a few cables to some grounding points in your engine bay directly from the negative terminal of the battery. This is commonly referred to as 'hyper grounding'. If you do a search, you'll find a much more comprehensive guide on how to do it, and if I get a chance, I'll take some pictures of what I did.. because I didn't understand at first either.. I'm a very visual learner.
