If it smells that bad then you most likely have a dead mouse in there. Usually after we clean out a nest in one of the trucks we use the air compressor hose in there to blow out all the other stuff (mouse turds, leftover nest, whatever) out.
Back to the dead mouse, you might have to gut out the whole interior (seats, rugs, everything) just to find the stupid thing. You might want to look in the air vents of your car while you're at it. And the traps and some poison are a must after all that, though I've only known to use a block or two of mouse/rat poison placed conspicuously around in the vehicles at my place.
I hate being spooked from hearing the snap of a mouse trap whilst I'm driving! :lol: ;-)
My 2 cents.