I know this is months later, but here you are in case you (or anyone else) needs the info again.
First, though, I feel that it wouldn't be responsible to neglect to tell you that if you got the seat in 2000, it is expired and no longer safe to use. Car seats generally expire after 6 years because the plastic degrades over time - in a 10 year old seat, it can look fine but actually be brittle enough that the entire harness can rip out of the seat if it's an accident.
For the manual, visit Cosco's website
and fill in the product number and date of manufacture. It will bring up a PDF file of the manual that you can download.
To rethread the straps, first replace the **** strap. Slide the metal plate down through the slot in the seat, with the buckle facing out. Slide the metal clip under the tab and push it down to lock it in place.
Facing the seat, start by threading the end of the harness strap down through the slot of the left side of the bottom (where the child's right hip would be). Under the pad/cover, thread it across the bottom of the seat and up through the matching slot on the right side. Pull it until you have equal lengths on both sides.
Thread the buckles onto the the strap (one one each end) and buckle them. Thread the chest buckle onto both ends, making sure that the harness doesn't twist. Buckle it, too.
Push the ends through the harness slots. Pull the harness through to the back, making sure that it goes OVER the metal bar, but UNDER the plastic adjuster handle.
There is a loop sewn into each end of the harness. The adjuster strap has a splitter plate, kind of shaped like a Y. Slide each loop over the ends of the plate and all of the way into the slot, so that the curved ends of the splitter plate are exposed and the strap is secure in it's slot.
Adjust the harness height by squeezing the metal bar on the back of the seat (the one behind the harness) forward and moving it and the plastic adjuster bar up or down to the correct position (at or below the shoulders for rear-facing, at or above the shoulders for forward facing). Do note that the very top slot is NOT for use with the harness - it's only to adjust the headrest when the seat is being used as a seat belt positioning booster and is not reinforced to support the harness in an accident!
Adjust the tightness of the straps with the button/extra length on the front of the seat. Properly tightened straps should lay in a straight line without sagging and you should not be able to pinch up any slack (lengthwise going down the harness, not across the width of it) at the shoulders or hips. Adjust the chest buckle to lie in line with your child's armpits, never his neck or belly.