Child Seat manufacturers recommend using the non-retracting hipbelt (middle seat belt) in older cars, OR use a locking buckle which usually comes with carseat to lock the seatbelt buckle, converting it into a non-retracting hipbelt.
Take the rig into a local fire dept or something and have em give a hand.
IIRC < 98 did not have ALR belts on outboard positions. If that is correct, then you would need to use the locking clip on the belt when you install the bases. Definitely use common sense and don't drive the kids around with sloppy seats until you get it fixed up!
It takes some fidgeting but I get mine snugged down tight in my 87 4runner by using the little "H" clip that comes with car seats. On one of the car seats that has a strap for the newer anchor type mounts I also ran that down to the seat hinge point for extra security.
__________________ 1987 4Runner, 22re, downey intake. 33x10.5 KM2s, 2" BL. Workin' on it.
Using that clip that comes with the car seat is for the outer shoulder belts. Our 2000 4Runner had the ratcheting shoulder belts and using those along with kneeling in the car seat when tightening up the belt worked great. The seat never came loose. I would think the same would be true if you fiddled with the middle one while kneeling in the seat. If you can't get it tight see your local fire department right away. There is nothing more important in life than keeping your kids safe.