Sub and Amp without a new head unit
 

I have a JL Audio W1v2 powerwedge and a Alpine M-500 amp and want to hook it up in my 09 fit sport but do not want to deal with the stereo just yet. My amp has the speaker level input feature so i can hook it right up to the speaker wires in the car and the amp will get a signal that way. I know the sound wont be as good as using RCA cables and a new head unit, but is it really that big of a difference? Also, are the speaker wires only run through the car frame, because I will need to tap into them somehow and I dont want to tap into them right at the speaker because I dont want to have a loose wire closed in the door all of the time.

It won't be as good but you will be fine with speaker level input.
I'm using that method right now and no complain.
If the GE is like the GD you can find the speaker wires around the rear door sills area.

Do you notice your lights dim during low notes or anything because the battery is so small?

I'm running an Infinity Basslink but no light dimming. Unless you running one of those megawatts amp or a few amps then you should be fine.

I'm running two kicker crvs and an 1100 watt inferno crunch amp on the stock hu no probs no dimming but don't leave the radio on with the car off and expect to be able to bump for more than 20 mins lol

how are you liking the basslink? i was thinking about gettting one for a long time but never seen any really good reviews