Mounting Bols for heater duct on 1998 Chevrolet S10

I have removed the entire dash out of my Pickup & need to remove the heater duct to replace the heater core ,BUT I can not seem to locate the bolts/screws that fasten it to the firewall , Am I going to need to remove the passenger side fender to access some bolts through the firewall ? Also does the AC have to be compromised to do this ? How many bolts hold this duct & where is the approximate location of each . Everything is so compacted & well covered that they are not readily visible .Any help is MUCH appreciated - Thnx

Asked by for the 1998 Chevrolet S10
I am sorry I work on some Chevy's not many, hopefully some one else can give you better incite but Autozone have some popular workshop manuals for free on line once you register (for free) on their website is a company that independent auto repair shops "buy" their information from that offer a very reasonable service to do it yourselvers. You get a one year subscription to year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... great quality information same as the professionals have ,much better than generic workshop manuals from parts stores, much cheaper than the factory manual. or try and get an online repair manual subscription.
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You have to drill through the casing to get to the bolts I believe there are 3. Go to YouTube and put in "2000 s-10 heater core removal" and you will see what I mean. I'm doing the exact same thing... what a bitch.