Don't know if this might help some people, but it's how I've temp mounted the amps in the back of my R33 for the time being, I was originally planning to leave the audio how it was until I had enough cash to do everything properly but after a couple of weeks with distorting bass and other annoyances this is the budget install i chucked in made up of mostly parts I had laying around. The final result is a 1500w rms monoblock powering 2 subs, and a 4 channel 50 wrms orion amp powering the crappy kenwood rears that came in the car. The stock run from the alternator to the battery in the boot isn't enough to properly power a 1500w monoblock fully but provides enough power to shake your nose hairs a bit .
Please see this page for installing your head unit http://www.groovesystems.net/ns2500/ Note that in this topic the author talks about cutting the stock wiring harness and putting in screw terminals, I recommend going to somewhere like autobarn/supercheap and getting a Nissan wiring harness. This way you can either get the right converter to plug directly into your head unit or at minimum get a Nissan wiring plug so you can hack that up instead of the vehicle wiring.
To start with I got a piece of MDF from Stratco and cut it down to size to fit my boot and amps i wanted to mount. Next came the challenge of finding somewhere to screw the actual MDF. I've never worked in a boot this small and it was a royal pita, eventually I decided the existing battery bracket should provide enough support to hold the MDF, the following photo shows the mounts used, both of the bottom bolts required new holes to be drilled.
You can see around the hole on the left a bit of black ink, I used this on the end of the bolts so i could mark out on the MDF where the holes had to be drilled. Got everything first time next pic shows the MDF mounted up. Note the dodge home made washers, it was late and the shops had shut so I couldn't get the proper washers, never let such a thing deter you from your goal
Surprisingly even after several heavy spirited driving sessions these 3 mount points are still tight and doing a great job, I was expecting another mount on the right side of the MDF to be required but it seems I was wrong. Mount your amps to the piece of MDF using what ever wood screws you have, purposely pictures of the mounted amps have not been included.
Here you can see where the RCA leads have been pushed from the boot into the cabin. The hardest part of this install was probably getting the back seat out, all the posts i read said to un-screw the bolts and rip hard pulling upwards until the clips popped out, I thought for sure i was going to break something I was pulling so hard. After several attempts I ran out of patience and just ripped up as hard as I could, sure enough the clips popped off.
From here the RCA leads are routed underneath the carpet to the head unit, remember not to run the RCA's on the same side as the power wires, the left side appears to have the hicas wiring and not power wiring. I could be wrong about this as all of the stock wiring was tightly wrapped in black plastic which made it hard to see which wires were through which conduits but I have no engine whine/ground loops so I guess I've made the right choice
The kick panel and door skirts shown in the next pic are held in by clips only, pull hard enough and they'll let go.
From here the RCA leads we're routed under the carpet, under the heater/ac outlet to the head unit. Connecting the power for the amps should not be a problem as with the battery in the boot you have very nice terminating points, I used a distributor block on the positive side of the battery and hooked both AMP grounds directly to the negative side of the battery. Note as mentioned earlier in this post if you want to run a lot of power through your amps you will need to upgrade the wire between alternator and battery.
And thats it! a late Saturday afternoon and a few beers later you've got a secure amp mounting point. I'm very interested to see what people have done for their own installs so I can do a tidier setup in the future, please post some pics if you have them