Years ago, me and my collie were on the way from Devon to Hants in my W124, noticed a smell like cow poo, didn't get any less after a couple of miles, in fact it was getting worse, looked to the back seat where the dog was and he'd had a very runny and prolific accident all over the (cloth) back seat.
Pulled off at the next junction to see what the hell I was going to do about this mess, by this time the smell was unbearable, and the drivers window was open, I stopped, got out, slammed the door and walked to the back to get the dog out, who meantime had jumped over the seats and out the drivers window (presumably he couldn't stand the smell of his own crap either) covering the arm rest drivers seat and door trim in crap in the process.
What to do?
Had to take my coat out of the boot, stick it on the drivers seat, drive to the nearest garage and pressure wash the inside of the car, dog and coat, got some very strange looks as you'd imagine.
The owner of J 972 GOW, a white 300D, if your car smells in hot weather, you now know why!