Black plastic blower housing on firewall cracked..on 95 deville, multiple cracks in multiple places, seems like it has gotten brittle over time. Hard to replace? Cost of item? Dealer item? Common problem?

 

Yes, 8 on a 10 scale i would say
It's a dealer part. call for a price. it's called a moisture barrier.
Yup

 

Common problem. You can only replace it with the engine out. Temporary solutions that have been discussed are patching / beefing it up with foil tape, silicone sealant, or patching type compounds.

The main problem with keeping the holes is the AC blower motor will leak air out into the engine bay, taking the path of least resistance, resulting in decreased air pressure through the air vents in the cabin.

 

Wow, that's a lot of work!!
Sorry if you went through that to change it but it's not necessary to remove the engine.