Brokenvaccuum line broke can't figure out where it goes

It has a vacuum pump driven off the engine, see if it is working.

I think what you are looking at is a VENT TUBE and not a VACCUUM TUBE

What is the YEAR/MAKE/MODEL of your Truck?

Chevy 4L60E ?

Possible damage within the key and so it is best to take a view from the service center if your duplicate key does not work. If so a new one must be ordered.

Here is a crazy possibility, but I have done this with a tiny screw dropped into a ford ignition (They use the same barrel).

First, find a bit of pipe that will fit into the barrel. The ink tube of a biro washed out might do, but it may be a bit small. I did it with a cut off car aerial being used as the tube. You might find that an automotive dealer can supply you with some kind of fuel line, or better still, a garden machinery shop that will sell you fuel line for a mower that might do. Anyway.... Find a tube, it will need to be a couple of inches longer than your key at a minimum.

Now, go get your cylinder vaccuum cleaner (or an upright with a hose connection, whatever)

Duct tape over the end of the vaccuum cleaner hose, then poke a hole in the tape for the small pipe to fit in. Now, duct tape around it to make a seal.

start the cleaner and slowly insert the tube into the lock cylinder, gently wiggling it about.

You see where this is going I hope. Say a little prayer. Good luck!

This code indicates a large leak in the EVAP system ( Vaccuum Leak ) you will have to locate a broken vaccuum line or check EVAP canister for a leak or crack. You can also take it to a garage that has a smoke machine and what they do is hook the machine up to your EVAP system it blows white smoke in and it will come out at the point where it is broken or leaking.It might be worth the money to have this done because it can be a nightmare to find a vaccuum leak and this way it only takes about 10 seconds and they usually don't charge much for this test.

I would check vaccuum hoses are all connected and unbroken.
Sounds like maybe when it "backfired" it popped a vaccuum line loose.
The way I check for Vaccuum leaks is spray WD-40 the entire length of each hose while it is running, when vaccuum sucks in the WD-40 it will "Rev" the engine some.

It sounds to me like your vent switch cable is either bound up, broken, came loose, or the vent switch motor is not working if its electrical. If its vaccuum assist, you have a vaccuum leak in you heating and cooling vac lines.

evap system to fuel tank

On your vette I cant remember if it is vaccuum operated or manual cable but either way get a manual for the car that has a/c heat in it . You either have a vaccuum line that broke or a bad diaphram or a broke cable. I am almost sure its vaccuum but check that first then look at diagram and slowly work your way from the engine to inside car. you can also try to listen for it with the car running. if its a cable operated then look underdash for the blend door and follow the cable to the a/c control on the dash . one of the two. if you cant find a diagram google it. does the controls feel any different now when truning them ?

The plastic line in the wiring harness goes to the heater controls; and believe it or not , the other end goes to the vacuum tank - but its in the FENDER !!!!! behind the ABS, kinda under the drivers side hood hinge - great place to put one ain't it ? No wonder GM is going broke !! Good Luck