You have the same truck I have... I skimmed through this thread so excuse me if I repeat something already said, I'll throw in my $0.02 on your first post:
Good Day!
I've got a 2014 Frontier 4 door, V6, SV trim pkg, no navigation or Rockford Sub but has rear camera & steering wheel controls.
I'm going to start this project by replacing the OEM speakers. I'll eventually replace the HU and add a sufficient amp vying for clean signal instead of the ridiculous claimed gigawatt models. I'd love to be able to keep the HU but it's impossible since the factory radio doesn't have pre-outs or ability to match the OEM preamp to an AM amplifier. I've been looking through Crutchfield's offerings and I'm looking at some JBL seperates. Has anyone tried the 6x9's from JBL for the rear doors? Any other recommendations out there? I'm also going to try to brace the door panels to reduce rattles & reduce some of the IM distortion artifacts. I've been away from car audio for a long time so any input from your experiences good or bad is appreciated!
1. OEM speakers are a 3.5" in the dash, 6x9" in the front doors, 6.5" in the rear doors. The front OEM speakers are wired together to the stock front speaker outputs with just a small filter in-line with the dash speakers - no actual crossover to separate the signal. A set of 6.5" component speakers up front will do wonders compared to the OEM combo, you can go 6x9" component but it's not that big of a difference sound-wise by itself. I'd suggest a decent set of 6.5" with a cheap adaptor plate. Recommendation for these is based on budget (as always), the main "issue" with the OEM locations is how the dash speaker aim up to the windshield instead of at the listening position. You'll still get better sound when you upgrade the factory paper cone speakers, though.
2. Rear speakers, keep it at 6.5", you don't want the rear to be louder anyways because it will pull the 'stage' backward, which you don't want. The plastic mount for the rear requires a cheap 6.5" adaptor since the OEm rear speaker is bonded to a plastic 3-hole thing, when you remove the OEM rear speaker you won't have a way to mount any aftermarket 6.5" without the adaptor. if you need more info on the adaptor I can look it up for you.
3. You can always use line-output adaptors temporarily to get the factory head unit speaker outputs down to line-level for aftermarket amps. The PAC converters are cheap and work just fine until you swap out the head unit for a good one with pre-amp out.
4. The Frontier doors are pretty noisy out of the dealer lot. a layer of Dynamat on the inside of the outer skin is one of the best things I did to my truck, it (combined with another layer of Dynamat on the inner door skin under the interior panel) totally changed the overall sound. Midbass kick is better, less rattling anywhere, less road noise, and your doors close with a solid "thunk" instead of a hollow metal sound.
5. I saw you mentioned "where to put a sub" - the easiest way is to get a prefab enclosure made for under the rear seats. You'll have to remove the storage bins under the rear seats, but it's pretty much the only place to put a sub (there's no room behind the seats). You can get a single 8" or 10" enclosure that sits on the right side of the floor, or a dual 8" or 10" enclosure that spans the entire width underneath the rear seats. Some people I've seen pickup a used factory RF sub that sits under the left side seat, but IMO if you're going to add a sub, might as well do a proper upgrade. Also, select a down-firing enclosure (subs are aimed down to the floor with a vent), the top-firing enclosures just aim the bass into your rear seat and suck up a lot of bass.
6. Steering wheel controls is actually easy if you have a compatible head unit, an Axxess ASWC-1, and the correct wires in place at the harness that sits between the head unit and the vehicle. There are 3 extra wires that give you full steering wheel controls, but most aftermarket adaptor harnesses don't have them in the bag.