what is that plastic tank on the passenger side fender under the battery tray that has what looks like a vacuum line and what is the purpose of this tank, its about 8 to 10 inches long and round at both ends?

Is it possible to post a photo of it please?

From what you are describing however, it could be a vacuum reservoir or emission canister, but unless GM rearranged things for 97-99, it'* in a funny place for either of those.

sorry I have no pictures, its under the passenger wheel well by the side turn signal, the charcoal canister is on the driver side, I know what that is for, but I cant get any info what the function of the small tank on the passenger side might be

I believe it is some type of reservoir but do not know what purpose or function it does

and it does look like a vacuum line, is it for the ac ?

I will try to get a picture

this is the tank that I don't know what it is for or what it does

I had to splice some vacuum line, the original line is a thin plastic material

If it is a vac line, and that is the only line connected to it(no electrical), then it has to be a vac tank......