In my opinion, RF modulators suck. There's so much audio quality loss in them, I don't think they're worth it.
To the OP: there's probably a few ways to go here, depending highly on your knowledge of simple circuits, handiness with taking things apart and putting them back together correctly, and how big your cajones are:
1. Open up your existing radio and see if one of the PCB's has a spot to solder in an aux plug.
2. Buy an aftermarket radio which has the aux (or even USB) plug.
3. Buy another stock radio, and perform option #1 just in case you f it up.
Nissan has used so many models of stereo headunits in the 2nd gen Frontier, it's not even funny. It's hard to keep track of what model is capable of what. There's two things to look for. Like I said in option #1, some of the headunits actually had spots where you could solder an auxiliary input in. They're usually screenprinted with an "aux" and "L+ and L- or R+ and R-". Here you can solder in a 3.5mm headphone jack, then just solder in switched power to the aux circuit, since there is a digital volume IC that switches between the radio functions.
The other thing that you can do (as some Navara owners have found), is that you can clip the AM radio input from that digital volume IC, and replace the AM radio inputs with you left and right AUX inputs. Then you'd just have to switch to "AM Radio" as your setting, which in reality would be your MP3 player or whatever. (With this option, you'd lose your AM radio.)