Start by pulling the glove box out and look around inside the dash from there. Just remove a few screws and it comes right out. Then pull the kick panel off the driver's side and look up inside the dash from there. I would avoid removing the brittle plastic parts if possible.
Have you turned on the A/C and heat to see if the smell is stronger from the air coming out the vents? If it is you can remove the blower motor and blower control module on the firewall and see what you find in there.