I have an extended cab Dakota and I'm taking rear speakers out of side panels and putting in 8" subs 4 ohm. I have an Alpine head unit which has preamp outputs for front, rear and subs. the door speakers are 6 1/2" 4 ohm. I also bought a 4 channel amp 4x75. I haven't put amp or subs in yet. enough power without amp I just want to bring up the bottom end more. I will hook the front and sub RCA connectors to the amp and run larger wiring to the door speakers and the rear subs. Is it ok to do this and just leave the fader to all front, or should I just use front and rear speaker out of head unit to amp and just use the 8" subs as rear speakers,to get more control over front to sub fade.

 

Leave the fader alone. Or fade to front. It doesnt matter too much as all the mids/highs will come out of the front door speakers via the 'front' outputs of the HU and all the bass will come out of the 'sub' outputs from the HU. Please dont use the rear speaker outputs for the subs, they wont tend to like the frequencies they aren't meant to play.