It sounds like what you have is an "ELR" (Emergency Locking Retractor: it only locks during a crash) and a "Free sliding Latch Plate" --the "male" end, that is...the shiny part that goes into the "female" buckle. (for lack of a better description, lol)
Cars prior to 1996 were not required to have locking belts, but since then, either the retractor must be lockable, or the latchplate must be lockable, to hold the lap belt taught, to hold car seats in place.
So what you will need is a "locking clip" to hold the lap portion of your belt tight. Almost all carseats come with them, and when they are used correctly, they do a good job of holding a car seat in place. (Britax seats and Safety Baby seats come with built in lockoffs, and no separate locking clip is required)
Here's a site with different types of seatbelts:
http://www.cpsafety.com/articles/lockincss.aspx
And if you have realplayer, here's a video clip of a proper locking clip installation
http://www.fitforakid.org/videos/index.html
Sorry, I just re-read your question...you didn't ask all that...but I hope it helps, anyway!