>Does the back seat
> have to come out entirely to get that plastic cover
> off the heater unit?

Yup. It does on my 85.  Not a big deal, though-- two bolts (12mm? I could
be wrong...my seat went in and out a lot for awhile there when I was
figuring out that the smell of "mouse pee" in the back was actually
antifreeze leaking....) on the side and then slide up and out. When putting
back in it can be tricky if you don't realize (like I didn't) that there
are little protuding ends in the back top of the seat that have to slide
into two little curved holders.  Not a big deal, but if you don't get them
in there the bolts ALMOST BUT NOT QUITE line up and you might start cursing
like I did. :-)  When in the little grooves, the bolts line up perfectly
easily.

Then there are justa few screws that go into the floor around the plastic
housing and it comes off.  (just in case of a leak and a lot of crud around
the bottom under the rug-- which has to come up in the back so you can see
the floor screws...and the rug don't come up with the seat legs in
place....)

BTW, I got my new rear heater core in Portland, OR at Halsey Automotive for
50 bucks.

--Patrice
85GL "VanaBlue"