Common problem with the Z, especially the earlier 240's.
Check your hatch seals, the taillight seals, and all openings on the back of the car.
A lot of times, even though your seals on the hatch are there, you need to make sure that they're being compressed properly. To do this you would need to adjust the latch mechanism, don't lower the catch too much or your hatch won't align to the rear fenders.
On the taillight seals, you're not just looking at the gasket that sticks out of the back, but rather the one that is getting compressed when the light is installed. The other "skirt" seal is to seal to the back panel fascia.
Also check the openings around the fuel tank and the spare tire depression, as well as the ones on the flat portion of the rear deck.
Common problem in the 1st gen Z's. Search the archives with "fumes" in the subject line, and you will find a ton of info. The aerodynamics of the car create a forward suction in the back, so the most likely culprits are usually the seals around the hatch and tail lights as mentioned.
Once your leaks, seals or gaskets have been corrected what I have found that makes a difference is to place the blower fan switch to any position other than OFF. I ALWAYS run my fan (low speed) because it seems to pressurize the car somewhat which helps keep those nasty fumes under control.
There is also a large hole on the drivers side behind the wheel on the rear inner fender that is the drain hole for the power antenna. The rubber gromet is usually rotted out so check this. Look close for any rust holes in the floor or missing/deteriorated sealer at body seams. There may be small gaps you never knew were there and thats all it takes for the fumes to get in.
In addition to everything else here, check the vinyl hatch panel on the hatch underneath the rear window: Remove it and make sure there is a plastic liner sheet with the perimeter sealed securely underneath the panel. If not, put one in or find 1 1/2" rubber plugs for the two large holes on the bottom of the hatch door.