Help needed with an electrical issue

I have an issue that has got me and a couple of others puzzled. On a 2008 Sprinter 2500, there are several things not working and I am thinking they might be related. Here goes:
1. Rear license plate lights not working (So far we checked fuses, changed bulbs and traced the wire back to the module)
2. Cargo area lights not working
3. Brake pad sensors on. (brakes are ok, and sensors seem ok too)
4. TPMS light on. (Sensors check OK after reset, only to come back on after a few mins. No blinking, tire pressure is OK)
I am wondering if anyone knows if there is a connection between these, maybe running on the same circuit? I am not familiar with the van's history as I just bought it last week. Just trying to get to the bottom of this, so any tips will help.
Thanks!

X X
Check the earth connections for corrosion, the earths are brown wires bolted to the body. Eric

X marks the spot... with a wire brush.

Thanks, will do! I'll come back and report.

OK, so I have only fixed one of the issues so far, the TPMS. Even though I still think the sensors are NOT position specific, after a rotate and balance job and another TPMS reset, the light is off. Been off for several thousand miles, so that worked.
I also checked the earth locations for the faulty tag lights as you guys recommended. It seems to be inside the cargo area, at the upper corner of the driver's side rear door, on a NCV3. Was a little rusty, not enough to loose contact IMHO but I cleaned it anyway. The lights are still not working. However, during a visit at the dealership for another issue they briefly looked into this as well and concluded there was 12V at the socket and the light bulbs were OK so more diagnostic time needed. I was in a rush so I postponed. The previous indy shop that investigated this issue for over an hour said there was no power on the wire, tracing it all the way back to the module/computer ? So that leaves me even more confused.
I have not looked into the cargo area lights not working issue, assuming it's related to the previous one. I will check all the light bulbs for the unlikely event that they are all burned out.

The cargo lights have the positive volts fed to them via one of the computers. They can be turned on from two sources.
1 . The light switch above the rear door.
2. If the the rear door depending on the position of the light switch in 1.

The light switch (and the door open sensors) has separate wires going to the computer, which signals the computer to turn the lights on. The computer also has the ability to keep the lights on for some seconds after the door has been closed and then initiate a slow fade out.

As a result, fault finding it is not as easy as just checking for volts.
You may find that the only thing wrong is the position of the light switch.
The light switch can be prized out and the connector on the back checked for corrosion and functionality.

Cheers

Ross