Hello guys,
Now if you modeled a car… or a plane… or anything…
how’d you build the interior?
My Way: Take the lower part of the object off (EXTRACT IT) and model the seats and stuff on it… then return it to the ship… (merging vertices back)
What would you do?
I would also in most cases extract it as you say, bottom up is easier for me for things like seats, for the dash I usually work top-down. But I would recommend breaking each larger object it into smaller pieces, like seat bottom, seat back for the seats (even separate the bottom sides from the middle if necessary). For a dash I start out with a box (or half a cylinder) that fits roughly the shape, then detach a clone of a section and then extrude or whatever you need to do to the pieces to create the extra detail components. I try to keep each of the larger sections welded together when you can actually see the seam, but a lot (if not all) of the detail pieces do not need to be re-welded together afterwards. For one it can be a real pain, and also will increase your edge count if all edges need to line up perfectly. So a lot of it is sort of “faked” (not literally bet that’s what I call it) in that it looks like one piece, but could be multiple sections aligned perfectly so you cannot see the difference. Most of this is highly dependent on how high detail the part needs to be, Lower poly work is much easier if the parts are smaller, when you need to deal with smoothing groups, the more complex the shape, the worse it will look sometimes, and breaking it up into sections is sometimes the only way to fix it. Also, sometimes it is better to start with a cylinder with a uniform curve, delete the ends and half of the cylinder, then start working from that instead of cloning a piece from the existing geometry, it all depends on the shape of the end product of course, but using pre-built shapes and cutting out a section that is useful to you can save a lot of hassle.
I also use “Instanced” clones of an object, where you can place one into the area you want to place a seat for example, start with 2 flat planes, place the first one into the area where the seat should go, rotate it so it aligns properly, then go back to your other plane and build the seat there (on the non rotated part). This will allow you to work on the seat with the world coordinates in place (move x axis on x, if it was rotated you would need to move X & Z to move a vertices to the same place), you can also switch from world to local orientation, but this doesn’t always work and can be a pain once the model is more detailed to move parts as you intended.
These are just a couple techniques I use in 3dsMax (but should work no matter what program you use), but everyone has their own way of doing things so this might not work for you, just thought I would add some of my methods. Let me know if you have any questions, I am currently working on the interior of a vehicle as well so I know how difficult it can be (I haven’t done too many car interiors yet so I’m still learning as I go), but once you get the basic shape down, it just comes down to adding details. Another good way to learn, is download some of the higher quality free models from various websites, one with a lot of vehicles, but some not so great, is http://tf3dm.com/3d-models/vehicles (try downloading the Lamborghini Aventador for example), or this one Free 3D Models and Objects Archive. Download: 3ds , obj , gsm , max models (again, mixed quality, but helpful) and take the model apart to see how they went about the issue, that is one of the best ways to learn some tricks to make it look good!
Hope that helps a bit! 
Interesting…
I’ve seen this done where it’s basically 2 models, one as interior and one as exterior, but that’s only for a fully enclosed prop (Car with windows up as opposed to a convertible).
Yes This is the same for me, but only after attaching all of the pieces together at the end, sometimes I will attach the seats and the interior walls together (but DON"T weld the vertices unless absolutely necessary) due to the reasons stated above. So I break down the car into groups (the dash/steering wheel is one group, the seats are another, the interior shell (armrests & the slots on the inside of a door as another piece, etc) on the doors, roof, back, and floor.
My car currently has around 200 individual parts that make up both the interior/exterior, which once I have created the low poly version will be attached together. Once complete I will turn that into around 20 sections (that are not welded together, just attached), and once I have my low poly complete (I call it Medium Poly… 
as I will be keeping it higher than I have in the past as the engine doesn’t have an issue with it unless it it ridiculously high poly and many need to be shown on screen). Before exporting I will create a group for each of the sections I need to have in UE4, so that I have 1 mesh for the vehicle, and one mesh for each of the 4 wheels.
In order to texture it all, and especially since I am going to have to be using multiple material slots (as there will not be too many vehicles on screen at a time, and Player vehicles have much more detail that normal traffic in my game), as well as I have some transparent, some non transparent items and would like to see how it all looks when done.
Again hope any of these pointers can help you out in Maya, 3dsMax is what I use, but the same things should be possible on Maya as well. Have fun!