To get decent sound and assuming you want to keep the RF headunit w/ Nissan XM receiver, IMHO, you might consider the following:
1. new speakers (there are many different brands to choose from). Many people use one or more component sets that include a tweeter, a crossover, and a woofer. If you go to crutchfield.com I think you can get a list of speakers that will fit. Others will fit if you make a mounting adapter.
2. amplifier(s) - the stock speakers are all 2 ohm and the RF amp is like 35 watts per channel so you need both speakers and amp(s). You need to match the amp(s) to the power requirements of your selected speakers and sub(s). If you are using high power sub(s) you might consider using a class D amplifier for the sub(s) because they work well at lower frequencies and have a high efficiency rating that will minimize the load on your alternator & battery. The stock alternatory is rated at 130 amps.
3. sub(s) & box(es) Most people use a box or boxes that will fit under one or both rear seats. You can buy a ready-made box, make one, or have one made. Some people make them out of MDF or MDF and birch plywood, and some people make them out of MDF & fiberglass. You can carpet them to match the carpet in the truck, cover them with vinyl, or paint the fiberglass boxes. The box(es) need to be sized so that they will (a) have a volume matching the requirements of the selected sub(s) for sound quality purposes, (b) fit under the seat(s), and (c) fit the physical dimensions of the sub(s). The sub(s) need to be selected so that they will fit in a box under the seats. Various people have opted to fire (point) the subs up, down, or forward.
4. LOC to connect the amplifier(s) to the RF headunit. The RF HU does not have RCA outs. So you need to use an LOC to connect the RF HU to your new amp(s).
5. maybe a pre-amp depending on the LOC as the RF HU puts out extremely low signal levels. I am hoping the UI-4 LOC from
www.altomobile.com will provide a sufficient signal level w/o pre-amp. It has nice remote amp turn on features also. It is supposed to be released this month.
6. a good 4-channel RCA cable to run from the LOC to the rear cab wall (assuming that is where your amp(s) are going) (a 16 ft. cable is good for a CC)
7. a good short RCA patch cable to go from one amp to another if you use 2 amps (e.g. a 4 channel amp for the front & rear speakers and another amp for the subs)
8. if you use a pre-amp (e.g. Audio Control Matrix) you need a couple more short RCA patch cables to go from the LOC to the pre-amp.
9. a distribution block to feed power to your amps (assuming you go with two amps). streetwires makes some fused blocks that also distribute ground
10. 0 gage high strand count power wire to go from the positive battery terminal to the distribution block and to do lizardking's battery-chassis-block ground upgrade and perhaps to go all the way to the distribution block ground per smoketitan
11. a gold plated negative battery terminal that accepts 0 gage wire so you can run 0 gage ground wire
12. a fuse holder & fuse for the positive 0 gage wire located right next to the battery (sized depending on amplifier power)
13. 4 gage wire to go from the distribution block to the amps & grounds
14. wire harnesses to connect the LOC to the OEM HU & factory harness. if you use two harnesses, you can solder all the connections to the LOC and then plug into the factory harness & OEM HU, w/o chopping any factory wires. The part numbers are Scosche NN03B & NN03RB. Probably other companies make them as well, they are sold under the designation "Nissan 1995 & up" or "Nissan/Infiniti 1995 & up". If you use both the regular & reverse harnesses, it saves chopping the factory wires. The reverse harness is the one with connectors that plug into the factory HU.
15. With 0 gage wire, I think you have to drill or enlarge an existing hole in the firewall. The wire should be protected by a bushing or grommet so that it doesn't rub against the edges of the hole and cut the insulation.
16. You either need to change the positive battery terminal to one that accepts 0 gage wire or use a ring terminal to connect the 0 gage wire to the accessory terminal.
17. You need to consider how the amps will be mounted. Many stereo shops will just run sheet metal screws into the rear wall of the cab, because that is the quickest thing to do. You can make a mounting board for your amps to avoid this.
18. Depending on your selected speakers, you might need to make or buy adapters to mount them. Many people make adapters out of MDF.
19. If you install a high power system with sub(s), you will get alot of rattles & resonance unless you use sound deadening (e.g. dynamat, brown bread from
www.bquiet.com, etc.). IMHO you should do at least the doors, rear floor portion, and rear cab wall.
20. You need to disconnect the OEM RF amplifier (I removed it and relocated the XM Receiver to the knee plate under the driver's side dash per schneid's posted instructions). If you want to use the factory speaker wires, you need to jumper into the factory harness to feed the speaker outputs from your amp. Eakes posted instructions on how to do this. For some vehicles, they make a "bypass harness" to jumper around a factory amplifier. I don't think they have this for the Titan. Alternatively, you can run new speaker wire. Using a huge zip tie, 12 gauge speaker wire can be pulled through the rubber door boots.
21. With speakers installed in the door, water (rain, car wash) will go into the interior of the door by flowing past the wiper strip on the door sill. The interior of the door is not water tight. So the back ends of the door speakers will get wet. If you don't want them to get wet, you can mount them in foam baffles (cups).
22. Some people make a sub box that fits under the rear seats and includes a place to mount an amp. One person posted pictures of a fiberglass sub box project that includes mounts for two subs and two component speaker sets.