The horizontal portion of the fresh air inlet plenum (directly below the air inlet grille and the plastic inlet screen) is essentially wide open for inspection - when the grille and screen are off. I think you're going to be okay as far as corrosion in the down plenums - and the cross plenum as well. I don't recall ever seen any significant rust inside the plenums. They are designed to flow water away from the windshield and one would have to assume that the inner walls are coated appropriately to handle that service. Plus, they lead a sheltered life - never seeing the UV rays from the sun, always protected from dirt, grime and stuff like car wash brushes. The paint usually looks like right out of the factory - like the under side of a sedan's trunk lid.
But, you're right, it is prudent to thoroughly inspect any steel that is adjacent to badly corroded areas.
The upper/horizontal section is easy enough to inspect; just pull the grill and screen. When they are out of the way, you can stand on each side of the truck - at about the door hinges and sight right down the middle of the plenum. A little shift to the left/right provides a good view of the front and rear walls. This is best done at night with a flashlight - or at least
not on a sunny day. Intense sunlight will cause your pupils to restrict, making it difficult to see into dark areas.
The vertical sections each have to be treated differently:
It's possible to do an inspection of nearly the entire height of the RH vertical section. On the other hand, the left hand side is less than accessible - and if the truck is equipped with factory A/C it's even worse.
The top of the RH plenum can be viewed by removal of the heater blower/evap coil casing:
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You will be able to almost stick your head into it and look around. It's not that open, but you will find it easy to evaluate the back and both side walls (inner and outer).
Even if you only remove the blower itself, you can still get a good idea of the condition of the upper portion of the chamber:
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With the inspection of the upper half completed, move into the cab to inspect mid to lower sections. Pull the fresh air inlet damper (or recirc damper if A/C equipped). With that damper and it's mounting frame out of the way, you'll have a clear view all the way back up to the fan inlet, the mid-portion and most of the lower reaches:
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The use of an inspection mirror is helpful - but not necessary. While you have the vertical plenum open, now is a great opportunity to reach down - as close to the plenum floor as you can get - and check for the presence of mice. They seem to love these areas (along with the cab corners) and will choose them over any other spaces - as far as where they want to build their communities. Run a hose down the plenum as well. If you can run water (down & out) endlessly, consider the plenum unblocked. If the volume of water coming from the hose can't run free - and causes a rising level in the plenum, the drain openings at the bottom should be considered it to be obstructed. Use a stick, compressed air blown up from the bottom or just keep running the water (and poking around with a stick) until the drain is clear.
Unfortunately, all the above only holds true for the RH plenum. The views allowed of the LH plenum interior are limited to the fresh air vent damper opening – and that’s only if you have a base (non -A/C) system.
If you have factory A/C, you don’t even get that. Since A/C systems function most efficiently when sealed off from the entry of outside air, there is no damper in the kickpanel. Removing the entire panel isn’t an option either. That’s because there is no opening in the steel cab wall behind it.
Basically, the only view down the LH plenum - on A/C equipped trucks, is through the very limited opening provided for the hood hinge recess:
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But, since the LH side is even more well protected than the RH side, you should feel confident about the condition of the left side. If the steel in the right chamber appears to have maintained it's coatings and shows no signs of excessive corrosion - the left side is probably even better.
