How dare you post those terrible pics!!! That reminds me of when I changed mine out earlier this year (heater core).
My avalanche looked just like that, brings back bad memories.
If I had to do it again for a customer, i'd honestly charge $1500 to change it out!
I started by playing around under the dash, then you remove the seat, then you figure out a few things need to be removed, then you remove the other seat, then the console (which on an avalanche sux), the the grab handle, then the onstar box sh!t, lower steering column, then you think you can pull the heater core (which is the hole damn assembly under the dash), now you pull the dash out some, jeeezz it turns into a nightmare.
Once you stand back and look at what you've done, you're like holy sh!t. Looks like a bad wreck, lol.
Anyway, back from my nightmare, hahah, no rattles on mine, I actually used a ton of wire ties and it's quieter than the factory did it.
Oh ya, i think i had a few screws left over, hahah.
Glad to see you got it fixed, i know personally it was a lot of fun, if I ever have to do it again, I'll trade the truck in.
later,
allen
Nelson Performance
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'94 Chevy Truck: 8.9 et, 408 twins