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I have a pioneer HU deh-p3000ib which i plan to install in my 99 civic and had a few questions about what to do about wiring the amps. I have searched but people seem to have slightly different questions, so i only got sort of an answer. Here are my questions:
1. I plan to install a jl 300/4 amp for my speakers for now, but i may add another sub and amp later if i am financially okay. This amp has a pre-amp output, but my HU only has 2 outputs and the rear one is for either the rear speakers or a sub. So, if i were to later add another amp, would i be able to use the pre-outs from the 4 channel to the mono and will i be okay?
Edit: Okay, so sonic lied to me and i guess theres no outputs . but can u use a y adapter or something maybe?
2. Also, for the power wire, i would only use 8 gauge, so will it be okay to run 2 8gauge wires, or is that "unsafe" and is it better to run a 4 gauge and distribute it?
3. Is 20 feet of speaker wire enough for the rears and front components? I have really bad skills at estimating approximate lengths.
Thanks for you help, time, and patience.

 
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1. If you're headunit has one set of RCA outputs, which you'll connect to your 4-channel, yes you can use the preamp outputs to connect your sub amp later. You will not, however, have any adjustability of the sub(s) from the headunit.

2. Don't know. Hopefully someone else will chime in here.

3. Depends where you put the amp. I think you will need more than 20 feet total of speaker wire. Wire is cheap, just get a 50 ft spool and have some left over for when you put in a sub.