I recently bought a 1998 328i and the same thing is happening to me! After driving for a few minutes I start to feel heat coming from underneath the dash on the driver's side. If I touch the plastic under the dash that is also pretty %$#@ hot. Today it was about 70 F outside and the windows were down and i could still feel the extreme heat on my legs even while going 50 mph. And my heat/ac is completely off with no "wind" coming from the lower vent.
I really need to figure this out! It's making me sweat while driving when i should be enjoying the new 193 hp at my fingertips!
If anyone knows anything about this problem give me an email or PM! Thank you!