[LH Lead-Foot]

Welcome to the shop / dealer tech world! Yeh.

The wire (Brown I think) at the alternator, key-on, engine-off, should have close to the same as battery voltage with a loss of voltage used to power the dash light. Wiggling the wire at the back, over the valve cover is your main clue. Although you're getting voltage, you can do the reverse and supply it with a ground jumper. Doing so, key-on, engine-off, the dash light should come on and stay on as the ground jumper is completing the circuit from the key switch, thru the dash light, to ground.
Now, if the light only comes on when wiggling the wire in the same spot or general area is the same results. So un-wrap the harness if needed, but find the BRN wire and follow it. It will most likely disappear into the firewall connector otherwise the "Black Hole of death", Just kidding! Since everything for an 1990 square body for service is on paper, having the "Factory GM Manual" will be helpful...but do not buy a Haynes or the other one, only GM will isolate the charging circuit, show harness views and show all of the connectors by number. By the 90's, GM was really good about info in the Electrical Service Manual.
But we don't have one. So to be helpful, I am 6'3" but on trucks, year ago, I would place a moving blanket or fender covers across the front fender without hit the radiator, and place a 1 X 12 about 3 -4 foot long, then crawl on top of it.
But the bulkhead connector should have a 7/16" bolt in the middle on the engine side, unscrew it, the carefully wiggle the connector, perhaps with some WD40 on it, but should come apart (Unless someone on this forum know different / these wonderful older trucks where not in my work bay often).

Remember, we are looking for the same brown wire, that goes to the alternator and failing the wiggle test over the valve cover pulls both ways, but most likely to the connector. The grease used from the factory on the connector side will be black most likely. But, we are looking for aftermarket wires, burn't-melted wires, cut-abraded wires or just a plan old loose connector at the bulkhead /firewall. You are on the right track. Remember, I believe their are grounds on the back of the drivers' side cylinder head, so a pinched wire, cover with PVC insulation maybe cut or grounded as well. Grounded will keep the light on so thats' not it. At this point, it is you, a good flashlight, WD40 to clean these 29 year old wires to find the brown one. Follow it both directions as pulling on it causes an intermittent should be visible.
Now, when I talk about "Pin Fit", is the male terminal fitting into the female terminal. Once you can hold it and see it, using another factory terminal you insert it and pull it out to feel if it has tension. How much? The same as 3 or 5 other terminals just like it. Each terminal has a tiny tab to use a small tool to flatten the tang, while holding the wire into the main plastic connector, will allow for the terminal to be removed. In the 90's, it should a 1/4" wide terminal with the tang in the middle on the top. So the shape of the connector with have a little window to flatten the tang for removal. Don't worry, they can usually be re-shaped, tang bent back up for re-locking, but in the center of the terminal will be the tension tab. From the wire side, a small tool can raise this tab to increase tension if it does not pass the "Pin Fit" test. The wire can have 18 copper wire threads, but if it has been brutally abused by someone else, it can have broken copper wires which causes resistance. You are looking for an open in this wire.
If both ends are out and visible, you can use you DMM set to resistance (Some have a Beeper option), but attach it to both ends, then pull apart with both hands every two or three inches of so. I have seen good insulation, but open wire touching inside, but open when pulling on it. Watch you meter or listen for the beeper to change or quit. There it is.
Having an intermittent is the worse problem to fix, but many vehicle are repaired using a new wire properly run from one end to the other.

You may choose to start a new "Thread" and ask for 1990, GMT400, alternator schematic as a member maybe able to scan a page from their book. Working on wires and the connectors are not too bad on the 1990 Trucks, but take some time to learn how to remove terminal from connectors.

I giving you a lot of things to do, so see what you can accomplish, because pulling on the harness over the valve cover and changing the reading is a good start.
Note; Plastic split wire loom cover can be purchased at home improvement centers and is usually cheaper that auto stores, but temperature rating, exposure are either good or crappy. If you have electrical tape covering the harness, if you have a fabric store near buy, go there and ask for a "SEAM RIPPER". These are cheap and the front is shaped like the letter "U". One tip has a plastic ball (It can come off), but the center is sharp. It is designed to place over thread sewn into fabric, they cut it and does the same on electrical tape as well. It will save time.
Best of luck. (See attachment)