One problem related to front seat belt buckle assembly has been reported for the 1999 Mercury Mountaineer. The most recently reported issues are listed below.
The rear 3 point seatbelts lock in the retracted position when worn causing the passenger to have to unbuckle the seatbelt to release tension in order to continue wearing the seatbelt. Both rear seatbelts continue to ratchet tighter each time the occupant leans back. This is extremely uncomfortable. The dealer has replaced the belts once, but the same symptoms persist.