I've posted this a couple of times, so I just copied/pasted from an older post. It might work for you if '77 is the same configuration.... don't know.
This is how I got the factory installed antitheft system working on my '75.
The first thing I did was to buy a wiring diagram of my car. I like the plastic coated (laminated) one from: Lectric Limited, Inc. http://www.lectriclimited.com. It's a nice working size and you can use a marker or crayon to trace wires. It comes in handy when you have loose wires and you don't know where they go, you can usually trace out the color and reconnect or find the other end of the wire very quickly.
1. There is flasher in the jack compartment that makes the alarm horn pulse. If it does not then that needs to be replaced. You can bypass this for a test. There is also a relay that you should hear click when you activate the alarm.
2. If the alarm goes off too easily, then I would look at the door and hood pin switches first. Get new ones and reset them - they are cheap ($2-3) from most local parts stores. If you have replaced the door weather-strip then the pin-plunger-stop is pushed in to far putting the triggering of the alarm on the "hairy edge".
The hood alarm pin switch is located in the engine compartment, on the passenger side, near the cowl edge.
The door alarm pin switches are on the lower rear door corner (at least in '75) and it includes the door ajar warning light on the gauge cluster dash (the interior lights pin switch are in the front near the hinges).
There are access panels in the front of the rear wheel wells. They are the 3" x 5" metal covers for the rear body mount bolts and are retained by 4 little hex screws. If you remove the covers, you'll see the wires within the door jamb, behind the switch. Might make working back there a bit easier. You usually have to take off the rear interior quarter trim panels too. It is a real PITA to work on the wiring behind these switches, but you can usually replace the switches without removing the access panels.
There is a wire running between the 2 switches along the front of the storage compartments under the carpet. Check that the wire is good and not shorted. Rig up a test light from one side to the other to check for a short. An open would not cause the alarm to trigger!
3. Check the alarm horn under the driver's side rear fender. Apply 12v to see if it works.
4. In the engine compartment on the driver's side fender, and it is a PITA to get at, is the keyed arming switch (lock cylinder). You will need to get at it from under the car by removing the carbon fuel evaporation canister first. You may even want to remove the windshield washer bottle (I did because the pump was not working anyway, so as long as I was there....). Like I said it's a PITA! Some people have small enough hands that they can get at the switch from the top, but not me.... and you have to be able to contort your arms to get a wrench in there, so I think it is less hassle to come at it from the bottom.
It is supposed to have a "anti-tampering" switch (looks like a headlight button switch, not the one on the dash but the one up by the light buckets). Sometimes this is broken and it can be removed. This switch is currently (2001) about $35 from Corvette Central and Zip. You can by pass this "feature" if you are short on cash. Personally, if you are going to all the trouble to fix the alarm, you might as well protect this area by buying the switch if you need it.
I spent the better part of two days, tracing every thing down and replacing the switches, etc. Now it works like the General made it. This system is NOT high-tech but it works! BTW: my problem was the tampering switch behind the key switch was broken (the alarm would never set and came on as soon as the key was turned) and the flasher was not working which let the horn blast continuously instead of pulsing.
On my car the flasher is a Delco #552 12V (looks like a normal turn signal flasher for that era, GM #491391) and the relay is a Delco #878 12V (GM #1115878 - discontinued). If you need to replace these and you are not NCRS picky, then try someplace like NAPA, AutoZone, JCWhitney, etc. Or possibly even an electronics supply house like Radio Shack, maybe even Summit or Jegs. I have seen these offered at swap meets, but how can you tell if the used part you are buying is going to work when you get it home? You sure can't return it if it doesn't work!
[Modified by KenSny, 5:23 PM 4/8/2002]
Ken - '75 L82-M21-FE7-J50-N41-C60-A31-N37 & modified
Medium Saddle Metallic exterior/Medium Saddle deluxe leather interior