Quote:
Originally Posted by GOLDCYLON
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Daryll........
I just pulled a heater core on a 1990 ZR1
WITHOUT pulling the Dash 
There is a little trick on removing one 7mm screws on top of the front vent connection (two 7mm screws). The first 7mm screw is easily removed using a 7mm combination ratchet wrench from underneath. The second top 7mm screw can be reached by taking a long (large) screw driver and pry out on the top duct and against the Heater Core housing top. That gives you just enough room to use a 7mm combination ratchet wrench followed by a needle nose to make the final turns on the 7mm top screw on that vent flange.
So there you have it....a new trick....one guy on Youtube said it was impossible but what does he know 
Quickly......as I recall......all accomplished mostly with a 7mm 1/4 inch socket and various 1/4 inch extensions 
Outside
1. Drain Coolant.
2. Disconnect Battery (I use a simple quick disconnect).
3. Remove two 7mm screws in Coolant Reservoir in front of Passenger side.
4. Remove two Heater Hoses from Heater Core and set aside the Coolant Reservoir with bottom hose still connected to Coolant System.
I had to cut the Heater Hose between the Coolant Reservoir and Heater Core to remove it.
Will go to Auto Parts with the old hose having a "U" bend and look at hoses finding one with the same "U" bend but longer and cut to fit.
Inside
1. Remove Passenger Seat (4 nuts and two electrical).
2. Remove hush panel two 7mm screws (and maybe one on right side also).
3. Disconnect light connector on Hush Panel and set Hush Panel aside.
4. Remove three 7mm screws at bottom of Heater Core Housing accessing from under right side Hush Panel area.
5. Remove one 7mm screw on right side of Heater Core Housing (half way down) accessing from under right side Hush Panel area.
6. Remove several 7mm screws back side of glove box.
7. Remove phillips screw front latch glove box and remove glove box.
8. Remove one top 7mm screw on left side that can be seen on Heater Core Housing from glove box area.
9. Remove one 7mm screw top right side Heater Core Housing that cannot be seen directly but use a mirror.
10. Remove bottom 7mm screw from Heater Core Housing holding air duct flange (there is a bottom and top screw).
Use a 7mm combination ratchet wrench.
11. Pry open a great space between Heater Core Housing and top duct with large screw driver.
12. Remove top 7mm screw from Heater Core Housing holding air duct flange.
I actually used a needle nose pliers to rotate the screw and lift out after it was loose using a 7mm combination ratchet wrench.
13. Now........wiggle out the Heater Core Housing down and out.
14. Remove bottom Heater Core holder screw and open Heater Core Holder.
16. Remove Heater Core.
Notes:
A. There are a couple wire holders held by fittings inserted in holes in the Heater Core Housing which you just slip out of the fitting and poke out holder once Heater Core Housing is removed.
B. The hoses to the Top Vent (one goes to the right and forward and one goes to left and rearward) have to be removed from the vent plastic tube.
C. The left Top Vent tube has to be pulled forward a bit (you can get a hand in the gap) as you pull down on the Heater Core Housing.
D. Make sure electrical connectors to the right side are not getting caught on the Heater Core Housing as you pull down on the Housing.
E. The wires that are under the Heater Core Housing have to be disconnected (3 or 4 connectors) to allow the Housing to be removed.
F. There is a vent tube connected (slip fit) to the vent from which the two bolts were removed in step 10, 11 and 12.
G. The Air Duct Flange has a nipple on it which helps if that flange is pulled from the Heater Core Housing at initial removal.
H. There is a 1/2 inch long flange (inside flange) on the Heater Core Housing where it connects to the main air tube to the left. That means it helps to initially pull/push the Heater Core Housing to the right when removing.
Replacement of Heater Core Housing will involve a magnetic 7mm 1/4 inch socket to insert the various 7mm screws
Actually pretty easy with the trick of step 11 to remove the 7mm screw in step 12.
See Item #1 and Item #2 Replacing the Heater Core and Heater Hose Connection