2002 tahoe blowing 40amp large fuses in fuse block under the hood

What all does that fused supply B+ voltage too ? Do you know ? That would be your first thing to find out ! Do you know what a electrical distribution diagram is ? Do you know how to test electrical circuits with a DVOM - digital volt ohm meter ? Do you know what a short to ground is ?
Which fuse ? IGN A - 40 amp ,under hood fuse / relay box ? This fuse powers the Starter Relay and Ignition and Start Switch
Does your vehicle have a manual or automatic transmission ? The starter relay could be shorted inside , It's also located in the under hood fuse box . If you pull the relay out of the box it's plugged into an look on the bottom of it you would see four sets of numbers ! 30 & 87 an 85 & 86 . If you were to jump 30 to 87 in the relay box the starter should spin the engine over , not start an run just spin ! 85 & 86 is the control side . Could be the ignition switch !
That 40amp fuse powers a lot of subsystems !

Here is a web site for free diagrams !
http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model an engine , Under system click on electrical distribution , under sub system click on power distribution . Then click the search button .Click blue links one at a time . First blue link scroll down to the second diagram . You will see Ign A 40 amp fuse at the top of the page .you will also see it powers the igintion switch ,look at last part of ignition switch , when in start or run that 40 amp fuse powers all those other fuse's plus three relays i see there also . A/C compressor relay , rear window defroster relay ,daytime running lights . Any one of those circuits could have your short . You need to isolate the circuits , pull fuse's till the fuse doesn't blow !

Starters will still work when worn but will have increased drag drewing too many amps. which will blow your fuses. before replacing older starter first remove all wire connections shiny clean and replace. make certain starter is tight to block(this is your ground. ) Important to remove ground cable from battery before working on starter.

That's usually a bad starter motor, shorted internally.

Blowing fuses is never an easy direct fix. Its rather a hunt needs some dedication. What I try and do is Figure out what is all on this 40 amp fuse, and one at a time remove something from the system and see if it blows. The key is knowing when it will blow for these tests to work. Keep removing what ever is on the system until it stops blowing a fuse. Then figure out how to single out items on that fuse. Whatever fuse it is the problem is on its path. Check all paths that go to items if no items are easily singles out. Replace as you find ,smash, green corrosion and shady connecters. Looks for heat by finding melted plastic connecters. These are clues. If you have a volt meter, then you will find the problem a whole lot easy, by figuring out what OHMS LAw is for whatever guage wire you are working with and a rough guess on length of known path. Using this you can find smashed and corroded wires quicker. As well as finding a short to ground and even short to powers in some cases. Good luck :)

Amazon.com or maybe RockAutoParts.com. Also consider NAPA Auto Parts, or a salvage yard!

That is a dealer only part.

It will most likely be a poor earth issue, make sure the earth lead between the battery and the body is good and clean and check the one between the body and motor.
Poor earth connections cause excessive current draw and will blow fuses.

Yes, there would be a fuse box inside car. Either driver's side at kick panel, or passenger side in the glove box or underneath at the kick panel. When you find it, put that 40 amp fuse back in and continue with your test. I don't know what that junction block fuse is for. Surely someone will post it.

Fuses only blow if-Old,Overheated or you have a Short circuit.
That should be any easy fix,to the extent that there aren't many 40 Amp Fuses and you will need a wiring diagram to follow the problem from the fuse to the cause. If you know what fuse it is, you can guess,but I wouldn't recommend that,it will be time consuming and maybe costly ,if you don't have a test procedure to follow and throw parts at it.
Don't replace the fuse.Their expensive and will keep blowing.

U HAVE A LARGE SHORT IN THE WINDOW HARNESS OR THE WINDOW MOTOR, START BU DISCONNECTING THE WINDOW MOTOR AND SEE IF THE SHORT GOES AWAY, IF IT DOES THEN REPLACE THE WINDOW MOTOR.