1998 Chevy Venture My rear air doesn't work

When you say that the rear a/c doesn't work, do you mean that the blower doesn't come on when you turn the a/c on or do you mean that the a/c doesn't get cold?

If it's the blower motor not coming on, check the rear blower motor fuse (in underhood fuse box--check the inside fuse box also for any fuse which may effect the a/c).
Check for power at the blower motor resistor (rear left fender--behind plastic side panel).