2003 Sequoia Electrical Issues (OBD, Horn, Airbag warning, etc.)

We got this 2003 Sequoia used about 15 years ago from a Toyota dealership. It’s my wife’s truck and we’ve been having electrical issues for several years. The first issue that arose is the gas gauge started showing no fuel when there was fuel. It would eventually correct itself, and we would rely on the mileage indicator on the overhead information display to verify the fuel level. Lately the mileage indicator has also been going to zero at times and when coming to a stop it will go back up. One of the worst issues is that when it rains the lights go on and the alarm will start going off. Water was getting in on the driver’s side and the floor would be damp. After some research, I added some sealant around the aftermarket windshield and the leak seems to be fixed, however, after several days of rain recently the alarm went off again even though the cab area was under an overhang, and it was dry. Recently the horn on the steering wheel and the OBD port have stopped working and the airbag warning light is on. Additionally, the power outlets on the passenger side and the back are not working. I found a thread here ( ) that mentions that the power outlet issue could be due to an issue with the IPO, and that is less pressing than the other issues.

I have checked all fuses in the cab and in the engine bay and they seem to be okay. I cleaned a couple fuse locations that were tight and replaced one fuse in the cab that looked like it had some corrosion on the pins. I also checked the connections for the battery ground, firewall ground, engine ground, and the right and left kick panel grounds and they are tight. The OBD fuse is getting 12v on both sides but there is no voltage when checking pin 4 or 5 and pin 16 on the OBD port.

I would really like to fix the OBD port, horn, and airbag warning if it’s something I can do myself. Are there any relays I should check or any other troubleshooting steps I can take? Visually, I don’t see anything that looks damaged from the prior leakage. There appears to be an optional wiring harness that is wrapped up and not connected in the driver’s side kick panel which is maybe for towing? I have reviewed the Toyota wiring diagram but I’m not that experienced with automotive electrical.

Sorry for the long post. Thanks in advance for any help.