If you're not yet familiar with relays, go to the top of this page where it says Relays and click there.....definitely worth learning if you're planning on installing more alarms.
Also, the Viper's install guide probably shows how to configure the relay to connect to the domelight.
Regardless, your question is a valid one.....it does seem weird, doesn't it?
Anyway, the output from your Viper (or most any other car alarm) is a low-current output that is very weak. It probably says something like 200mA.... 2/10 of an amp. This probably wouldn't even be enough to drive a single overhead light in an old economy car.....let alone the multiple bulbs in multiple locations that GM usually uses.
If you hook the dome supervision output directly from the alarm to the lighting circuit in the car, it might work once if you're lucky....but will almost surely burn out the alarm's output.
However, a standard relay's coil only draws around 1/10 of an amp...the Viper's output can comfortably drive at least one relay, and usually two or three.
So, look at the relay's pins:
85 and 86......when these receive power and ground at the same time, the relay is "engergized" or "on." When not, the relay is "at rest" or "off."
Then you have three pins going up and down the middle: 87, 87a, 30.
At rest, 87a and 30 are connected together (inside the relay of course), and 87 has no connection to anything.
When the relay is energized, 87a is disconnected, and now 30 and 87a are connected together.
As you may be imagining by now, you can use these three middle pins in lots of creative ways, activating (or deactivating) things as desired.
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IN YOUR CAR:
86: BLACK/ white domelight output from alarm
85: constant power
87: ground
87a: nothing at all gets connected here
30: white domelight wire in car
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HOW IT WORKS:
When everything as it rest, the relay performs no function, and the car stays just like it was from the factory.
When the Viper system puts out a ground on 86, the relay's coil energizes (because you've already connected positive to 85).
This causes 87 and 30 to be now jumped together inside the relay, which applies chassis ground to the car's domelight wire.
As you can see, the only item with the heavy workload in this case is the car's chassis ground.............the alarm only does a tiny bit of "work" to tell the relay to turn on........and then the relay only does a tiny bit of "work" to jump 87 and 30 together.
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POSSIBLE SHORTCUT:
I've done this on my cars, including many GMs, but never on an F-body like yours.
The car's BCM already has some sort of a relay built in. The relay gets activated by the car's two doorpin switches, by the trunk pinswitch, and by the dimmer wheel on the dashboard.
It's very, very likely that these items are low-current, and don't draw more than the 200mA your Viper alarm puts out.
If you can pull out the headlight switch, try to find the wire behind the dimmer that turns on the domelight.....it should test as a ground only when the switch is moved to "on."
If you can't find it there, go to either of the door pinswitches.
You can probably (but it's hard to promise) connect the Viper's BLACK/ white directly to the car's domelight-switch or doorpin wire without using any relay at all.