hi your horn is defective ! there could be a short but it is widely known that a horn could still honk before blowing fuse,
if the vin codes match then they should but i'd look at the wiring diagrams to be sure...
I would check in the engine compartment around the fire wall for a broken vacuume hose
I would suggest checking out the Haynes Repair Manual from your local library. It is a very helpful resource. I am using it for a 1998 Transport and Montana.
The gas tank in my extended 2000 Pontiac Montana is 25 gallons. I would imagine the 02 is the same.
A common problem with the Montana is the power steering pump.
I have a 2000 Pontiac and my was doing the same thing. It turned out to be my heads were warped. You can smell it coming out of your tail pipe if it is your heads.
check the cabin filters inside the glove box, if theyre clogged that will cause low heat it happened to my wifes 2000 silhouette
first suggestion would be the head gasket which Montana's are notorious for having problems with, followed closely by a crack in your cylinder head or engine block
Blown fuse, or defective headlight bulb.
check the ignition modulator under the distributor cap
lower intake gasket
Possible starter or starter solinoid? more info would help