Exhaust fume smell when driving with open window

Leaking exhaust maybe and / or faulty cabin vent outlets.

The car may have some cabin ventilation that normally only outside air can get in but maybe theres a pocket of exhaust fume where it shouldnt be underneath, or maybe leaked fumes rising under the bonnet and reaching to wherever the cabin air gets in.

You having the window open probably creates low pressure in the car sucking air in through any vents, possibly even the ones that should go outwards.

There are probably vents that should go outwards at the rear of the car somewhere, so that when you have the blower on air can pass through the cabin, without them the blower wouldnt work well.

Maybe those vents should have one way flaps on and they might have failed. This is exactly the setup on a Carlton for example, with outlets with flaps close to the rear exhuast silencer.

My elan used to do this, extendiing the tailpipe further rearwards fixed it.

Thats a good one! It is in bad shape!