This will be tricky.
I would avoid upping the ISO at all costs. If you do only go up one setting say from 50 to 100 or if 100 is the lowest setting go to 200.
Use the largest aperture possible. Probably F2.8 if you are using the FC-E9 lens.
You may have to use shutter speeds up to 8 seconds long depending on how much light you have to work with. Definitly turn on every single "mood" light you can.
You might consider using several flash light (2 d cell) and put them on the floor in locations that cant be seen by the camera. Point them at areas that are very dark. Cover the flash lights with some white paper to get the light to diffuse more so it wont make a hot spot and to lower its intensity. The idea here it to create "mood" lighting not "spot" lighting. Light that is spread across the floor and bounces off the sides of the interiror cabnits to simulate what the actual lighting does inside the limo. You should study the light available in the limo before you do the shoot and test out locations to put the flash lights.
The inside of the limo probably has overhead lights used to make it bright inside for cleaning purposes or just so people can see when getting out. You can mask these lights with some diffusing material to reduce the intensity.
You should take at least 5 or more shots using different shutter speeds and one with an increased ISO. I would bracket these at 1 stop apart. To get the really dark parts and shadows to have some detail will take a long exposre and this will blow out all the lights or at least make them all little solid white blobs which dont look natural at all.
You can then use HDR to combine them into one image. This image will have much more data to work with than a single exposure. Now you can tweak the image in photoshop to bring out any shadow areas you need to.
If you will have people in the limo the will have to be absolutly still. So make sure they are sitting in a comfortable position and understand why they have to be still. ANY movement at all will make them blur especially at the shutter speeds you will be using. If you do have people in the limo I would limit that 2 or 3 people at the most and make sure they are dressed up and have some props in there hands like a champaign glass or something. A really pretty cocktail dress would give a focal point of interest and make the scene come together. Set the camera close to this person.
One last idea is to do the shoot at dusk so you have some natural light to work with. This time should still make it seem dark inside the limo but allow areas around the windows and top of the seats to have some lighting from the windows. It may also make the veiw out the window have detail if there is a susnset or something. This would be better than a solid black window.
When you finsh the shoot post back so we can see how it came out.