Brake lights work, but tail lights not o

First thing I check is the fuses - test them with a DMM Like this, though the 4,000 count version will work if your budget does not allow for the 6,000 count version. Both are TRMS. Check the fuses for continuity by putting the leads on the metal tabs. If it doesn't beep or show 0, the fuses are bad. Change them and see if the issue comes back, and while you are there you probably would benefit from spraying automotive safe contact cleaner like CRC QD.

If the connections look good in the fuse box and the wiring to the headlights appears to be in order, I would locate the fuse box disconnect and spray contact cleaner on both sides of it. The problem is that, over the years, as you drive these, sometimes dirt or debris builds up and causes issues. You do need to disconnect your battery for this one, but the others are low-risk enough not to require it explicitly. I would also spray the ECU connector while you're there, if you feel the need, and it's in proximity to the fuse box wiring. It could also be a bad relay, so I would check those over as well. My suspicion is it's an intermittent relay or a bad fuse.

If both of those are good, you likely have deeper issues, such as a switch or if it’s anything like the GMT800 the controls are on a stalk assembly. Hopefully, this is a 1996, so you have OBB2 and can use a cheaper scan tool with OEM data, IF the need arises. For yours, you will need something like the Innova 5512 or 5610, along with an OBD1 adapter that Innova sells, as OBD1 cars do not have a standard connection. That is to say, GM uses a different connector than Ford or Mercedes. You can skip this for now, but if it's extremely persistent, it wouldn't hurt to check for things like BCM codes.

Before buying a scan tool one thing I have had success with old GM cars is before they put everything on the infotainment system and decided to mandate OnStar with the Google Voice only using the AT&T cell connection (and not the phone, except for Android Auto until the 2026 ICE cars lose it too) is they are prone to wear and tear, which can sometimes be fixed with contact cleaner - other times you just need to replace the switch if the relay is good.