How you rebuild the key cylinder for an 88 rex? Can they be rebuilt? or do I have to just go out and buy a new one.

 

Once they are worn out, your friendly neighborhood locksmith can fix you up.
If you do the hard part & take the lock assembly(s) out & carry it(them) to him, it's not very expensive.
Call first for price & availibility.
Ask him how to lube it so it lasts longer.

If your key is about worn out also, you might want to get all 4 cylinders re-keyed.

 

Here's a pretty good write up I came across on a Mr2 forum.
http://www.padandwheels.com/mr2/tails/lock.html
In any case the procedure should be pretty much the same.