cabin lights and seat-belt warning light stay on

Just went to start the car (96 caprice wagon) and realized battery was dead. The dogs are always clicking the rearmost cargo lamps on, I assumed they stayed on without my noticing and drained the battery. I pulled the bulbs to prevent this problem from happening again. Boosted it and let it run for a while, decided to get groceries done while it was just idling. When I was driving, I noticed all the cabin lights stay on like when the doors are open, I also noticed the seat-belt warning lamp stays on even when belt is buckled. I first checked that the dimmer wheel is not rolled to max setting: it's not. I opened and closed all the doors and tailgate to make sure none were ajar, the problem persisted. I even tried turning the car off and on again, problem's still there.
Cabin lights stay on when the car is on or off, belt warning light goes off when they car's off though.
I gave up and pulled the fuse for the cabin lights for now as a band-aid so I don't have to boost the car every time I go to work.
Any suggestions where to start looking would be appreciated, as my electrical skills are very so-so. Thanks

So with the warmer weather and some time off, I finally decided it was time to fix this. Figured I'd report back in case someone has similar issue in the future.

I removed the bottom trim and rolled the carpet back in the driver foot well. There's 2 bundles of wires running back along the side containing the wires to the seat/seatbelts.The wires from the door switch join into one of these bundles closer to the fuse box.

From said bundles, there's a single wire marked for SIR going to a ground under the hood release lever. There is a pair of wires going to another ground a few inches away. My guess is the second one was the culprit, even though they both looked fine.

I unplugged the battery, undid both grounds, sanded away with a rotary tool and found there to be a bit of corrosion underneath the paint. Polished everything off, put it back together, and behold the problem is fixed!

Glad you got it fixed and now you have me thinking that's whats wrong with my wagon.I have a aftermarket keyless entry with remote start.It will occasionally act up and not unlock the drivers door lock while everthing else works fine.Now that I know where the grounds are and the fact that with my back I can still get to them easily I will check them out.Thanks for posting the "possible" fix and those great pics.