yes could be a loom fault, i assume when your outside the car the door is open? could be a fault in the wiring or connector going to the door and when you have it open it works?
try sitting in the car awith the door open to get it working or the opposite, close the door and lean in through the window and try it.
this will tell you if its in the door harness possibly.
the other possibility is a wire is trapped possibly under the seat? so when you sit down it crushes it shorting it?
have a look under there and check
other possible faults are water ingress which can cause all sorts of funky problems as it shorts out connectors.
i wouldnt have thought the windows and seats are on the same part of the loom tbh, they even come up from different harness runs into the BCM.