As I understand it, there are solenoids which, when energized, allow the
seat belts to be extended or retracted, and which when they lose power,
while lock the seatbelts in their current position.  Because the current
neccessary to keep those solenoids energized can drain a battery quickly
there is also a Seatbelt Control Timing Module (SCTM) located under the
middle of the front seat which will de-energize the solenoids after 30
minutes of receiving the last signal that says the seatbelts should not be
locked (i.e. a door is opened).

Because it's both seatbelts, I would start by looking for a bad fuse.  You
might also disconnect the SCTM (disconnect the battery first) and see if
that helps.