use an enzyme based cleaner (like Nature's Miracle or Kids'n'Pets) that are especially designed to help clean up stinky smells like pet urine, sour milk, spitup, etc. You can find them at pet supply stores, I used to get Kids'n'Pets from Target but haven't seen it there lately.
Check your Britax manual about how to clean the straps. *PLEASE NOTE*: This is what a Certified child-passenger safety tech says about cleaning the harness straps:
"Why can’t I wash my harness straps or soak them in water?
Soaking the straps will remove the chemicals that make the harness fire resistant. The water won’t damage the fibers.
Using detergent on the straps can weaken the harness fibers. Also, putting them in the washing machine can stretch the fibers if they get caught around the agitator. Never iron the straps to try to flatten them or use any other chemicals such as a fabric softener, bleach, starch, etc.
If in doubt about how to care for the harness, use a damp wash cloth and wipe it down. A toothbrush is great for getting in the little crevices of the straps."
(from http://boards.babycenter.com/n/pfx/forum.aspx?tsn=1&n... "