Nice ... I'm also a fan of the bumper mounted fiberglass antennas ... though, for Xterras I think a no-hoop bumper (specifically, I'm a fan of the Shrockworks' lines) works quite a bit better with the look of the vehicle. The ARB works so much better on that body Frontier, than on the Xterra, for whatever reason ...
I put my CB in the same spot, but on the driver's side ... I already had my Mag-Lite mounted in the spot your CB is. Another good radio to look at is
Midland 1001LWX ... same form factor (compact), but adds weather to the Cobra's standard features. There's also the
Cobra 75 WX ST ... but I'm not a fan of having all (or at least most) of the radio gear in the handset. I went with a full-frame radio (really wanted weather functionality and that Midland wasn't out when I purchased), and in retrospect kind-of wish I had tried harder to stick with a compact unit ... in our trucks there's just not enough room to cleanly mount a full-frame unit. Here's the one I have:
http://www.rightchannelradios.com/uniden-bearcat-pro-538w-cb-radio.html
Right Channel Radios is great on selection and customer service (and tech info), but not always the cheapest ... I've just used them for the links here because I know their site, and knew what I was looking for ... some diligent shopping around can turn up better deals if you know what you're looking for.
As far as the grommet, what are you having trouble with? Is it the size of the PL259 connector? ... if so, consider looking into an SMA terminated cable and using a PL259 adapter, that may make it *much* easier to run the cable ... either that or a "solder yourself" connector. I don't know if the Frontier is the same as the X, but there's a really easy to access factory grommet on the driver's side firewall ... drops down in a really convenient place in the footwell, too. That's what I used for my antenna wire ... there's an equivalent on on the passenger side, but it's harder to access on the engine-bay side because of all the fuse/breaker/relay boxes.