This is from the state highway patrol and what I've always gone by:
"A handgun is concealed in a vehicle if it cannot be readily seen by a person approaching and if it is readily accessible. "
If a seat belt or door are obstructing the firearm, I would say that it is concealed.
I don't know of any statute that clearly defines when a firearm is or is not concealed. The AG used to have a bit in their NC Firearms Laws booklet about IWB holsters being considered concealed even if the firearm is openly displayed. I don't see that in there any more, so maybe it got challenged?