Main problem when I start "kitbashing" cabs using the actual body mesh is the interior consoles intended to be visible from outside, along with the lower poly simple texture inside the cab itself. Along with that comes some models you can't even kludge around, windows that are opaque from the inside for example. If model creators would take the main mesh, strip out the insides so all you had was a basic cab shell and export that as an additional mesh, it would be easier to add control components without having to work around the clutter.
Not sure what Euphod is referring to "restricted", but for many of mine I've started removing the outside cameras and using PEV attachment maker to add a.outsideview0 (1,2,3 etc.) to the model itself, so when you cycle thru whatever interior cameras are in the cavbview itself, it automatically switches to the exterior attachments and cycles thru those before returning inside. Suggested downloads;
brakeman Oil Tanker XP,<kuid:522774:100138>
ChiMet_GP9 650,<kuid2:522774:100116:1>
CNW bi-level commuter car 1,<kuid:522774:100408>
CNW bi-level push-pull cab car 1,<kuid:522774:100409>
CNW F7A 1,<kuid:522774:100404>
Select any one of those, hit the 1 key, then [ and ] to see what those look like.