Steve Cox's UV Mapper 0.2.9 for the Mac is what I'm using to generate texture maps. I've downloaded a good looking low poly 'sports car' 3Ds Max model from turbosquid. It has a pretty good hierarchy: all four rims are bunched together, for example, but they'll all need the same texture. I've built some decals in Illustrator and successfully applied them, (the flames are backwards, but that's easy to fix) only after isolating the 'metalbody' object, saving it as a dot obj, opening it in UV Mapper and generating the map and obj files. I deleted the original model 'body' and replaced it after making careful notes of the location of the original body.'s location in the Carrara file. The sports car came with a UV file, but it doesn't map to the right co-ordinates...it makes a mess.) Is there an easier way or must one generate a new dot obj file and paint (suppose those chrome rims are dirty) each piece of the model separately? (I realize I can build shaders for the car in the original file in Carrara.)

In the vertex modeller, try mapping each group parametrically (a.k.a. flat mapping), unless the group is topologically similar to a primitive (cube, sphere, cylinder). If you're using UVMapper Mac 0.2.9, you can make one of those "multi-layer" templates that includes all the parts, and you can delete groups from the "multi-layer" template to save individual templates for each group. ISTR there's no "Delete" command in UVM 0.2.9 (one of the many reasons why a UVM Pro version is needed), but you can shrink and drag parts off the screen to simulate deletion.

I am experimenting very hard with UV Mapper on the Mac. I found out that saving a complete GROUP as .obj (i.e. the whole body, including chrome of the car) and importing it in UV-mapper gives you all you want in one texture file. The only thing is, you should give them all different names. (So a left and a right version of a tail light should have according names)
Selecting all and shrinking it (shift "-") gives you the opportunity to select-per-item and nudge them to a specific place.
It's is like making wooden puzzles in kindergarten all over again!

Nowadays I sometimes make duplicate versions of parts, one version to texture, one version to remain in place to show where additional dirt, exhaust tracks should be placed.

After making a UV map like that one can choose how to export. "Export as one group" is very nice, because you only have to apply one shader to one combined object in Carrara.

Still, to put the whole shebang in one texture map (like a complete car), seems a bit unpractical. But combining, like, all chrome, can be a good idea.
To make things really realistic dirt and rust should have a certain direction (i.e. where would you expect those crushed flies...), sometimes that takes some experimenting, because once you rearrange the parts in UV Mapper you cannot see clearly what is back or front. So some back-and-forth swapping of simple texture maps (just colors) to see where a particular pixels ends up will occur. To prevent painting the best crushed insects you ever created, and later finding out they will spend their live (well, their death in this case) on the BACK of the grill.

The short version of my answer:
No, you don't have to generate new .obj files. You can export complete groups.

Thanks to mateo and hoofd for your valuable advice. I'll play puzzles with the car later today. I'm beginning to learn that one HAS to have the patience to render, render and render some more in order to see what's going on. For example, the oddity on this car where the 'bug bra' might be...maybe that's what it is, with the model maker's name on it. Hoofdcommisaris: how huge a file in pixel dimensions do you use?

1280 x 960 is my standard export preference, but I try to remember how big the texture will end up in the final render. When you put loads of parts in one texture map, the definition of individual details can be lost. When it is just the rearview mirror ( in which things may appear closer than they are, as Meatloaf observed so beautiful ), you do not need that much information. The fun with UV Maps is, that if you scale down the whole thing, the relative placement of the pixels stays in place. I.e. if you scale down to a half, that 'Penzoil' decal still will be in the same place, when you re-open the file in Carrara. The practical part is that it keeps your file lighter and the render a bit faster. Half the time things I make are meant to be printed professionally (dimension as in print, resolution 300 dpi), so the texture maps that are really close should contain enough pixel information to hold up to that. The car is looking rather Fast&Furious already! Try to insert a 'plane' primitive to get some shadow (contact with the ground), that helps to 'feel' the mass of the car. Good luck! Keep rendering.

Firstly, I forgot to credit Simon Moir, whose model is offered at Turbosquid.com-terrific, EXPENSIVE pro models. I've not messed with lighting or placing this car yet-just threw up 4 bulbs. (That must have hurt! HA!) The flames are starting to look like they belong. I used your 'grouping' suggestion and was then able to use the uvm dot jpg file that Simon included: I couldn't come close to it in any mode, planar, box etc. I look forward to the completion of my own little car model.