How would you actually shower in a bag? I see two options:
First - you have the upper end of the bag loosened up to allow you pouring water from a hose or a showerhead on your neck, shoulders, possibly even head and for it to run further down your body already inside the bag, so it would look something like a cone-shaped sleeping bag, but then you'd definitely have to stand (or in the worst case kneel or sit) for the water to efficiently run down the body.
For such system to be the most efficient, the canister would have to be under your legs, you'd basically have to stand (or kneel or place your legs on top while sitting) on it. In simple cars there's too little space for this, it's possible in a van or a similar, bigger car, but in a van, you'd rather make a better stationary shower like here, rather than using a bag. Also, to keep the upper end of the bag loosened while you pour yourself over with water, you'd have to mount it on some frame, use ropes, etc.
Second option - "shower" in other than upright position. In simple, low height cars, you could try showering half lying, with the canister placed at the end of your feet, but this would be more like bathing than showering, the bag, if it was tightly sealed, could fill up with water and you'd soak in it like in a bathtub, the hose providing fresh water would be connected somewhere along the shoulder line or in this case anywhere would perhaps be good enough.
Once you've soaked in a water-filled bag for a while you could drain the water to the canister by opening a valve and repeat such "showering" few times in a row. But how would you soap yourself in this case? In the first option, the loosened bag would allow you to rub soap on your body, but in the latter case there's no space for that, it would be super inconvenient.
To summarize, it would be easier to just drive somewhere remote enough and shower outside by using a portable shower that you hang on a branch or similar means. If it's really cold outside, then maybe a car is a better option, cause you can heat the interior up, but in a low-height car, it would be very inconvenient to shower using a proposed system. The cone-shaped shower bag I described in a first option might serve as an alternative to a more stable shower in a van or other big enough car, but it might be a headache to set such a system up and dismantle it after each time.