Originally Posted by Waveho

If you've replaced the windshield cover already, you've done about 1/2 the job; check out your lower flashing and you'll likely see some major cracking issues too.

Also, remove your engine cover and see what's left of your cowl--the burning rubber you smell when you turn on your air is likely caused by the deterioaration of this rubber cover which encapsulates the air ducts from your engine compartment leading too your cabin.

For e39 540i:

cowl part #: 51 718 157 446 ($32)
cowl flashing/lower windshield strip part #: 51 718 159 292 ($72)

tools needed: screwdriver (for prying); 13mm socket; 15mm socket; phillips head; rubber mallet and some tool (for removal of the driver's side wiper control arm).

The tools are for the flashing and wiper arm removal; no tools are necessary for the cowl replacement.

Here are some pics of the compartment where the cowl should be, but in serious disrepair and mostly missing. Compare this pic to the "after" pic of the engine compartment below! Also, I'm pointing to the remaining portion of the cowl in one pic. And in another pic are the new cowl and the stuff I pulled out of the compartment that WAS the old cowl: