I have tried to locate exactly where there is a leak in the exhaust but found no such air leaks. The smell is apparent mostly when the heat is on or fan is blowing. There is no sulfur smell coming out of the back of exhaust, only in the interior. Even if there is a problem with the precat, shouldn't the smell be outside? Even on highway where I thought the exhaust smell would be behind the car, but still is entering the interior.

my buddy had a problem with an oil leak and it was most prevelant when we were on the freeway small leak hitting he bottom of his exhause flange (85 S-10) so dont rule every thing out untill youknow you are right

You probably have a bad cat.

This might be out of the box, but don't they put an odorant in R-134a A/C systems. I heard this is done because R-134a is harmful to you (but safe to the environment... gee thanks). I have no idea what the smell is like but it may be worth checking into. If you have a slow leak in your A/C evaporator it would leak into your interior.

If you've checked everything else on your car, try checking the A/C system. I would think a Mechanic with a R-134a leak detector could check this for you.

I hope this helps,
Gears