"Is the carpet different for 4-cyl. and V6 cars, even though both are notchbacks?"
The carpet is different for cars that have the fan and cars that don't. 1988's do not use the fan therefor the carpet is the same for 4's and 6's.
1984-1987 (2.5)
1988 (all) TRIM, rear compartment (silver gray)
20583401 $127.00
1985-87 (2.8) TRIM, rear compartment (silver gray)
20608008 $127.00
"Is the air duct supposed to be behind the carpet"
Yes.
"How about the fan, behind the carpet, right?"
Yes.
From: Scott Backer