What is this ?

A rubber tube inside both doors of my 03 ZR2. It runs up the door piller, along the head liner and down the back of the hatch compartment.
Thats as far as I followed it.

Anyone have an idea what it's for ?

http://www.baja.chaching.info/Picture_0031.jpg

It's just a drain tube to re-direct some of the water that naturally goes inside the door in the right direction.

You'll want to clean them whenever you have the door panel off, and also clean the drains in the door from time to time

I'm not sure that I'm understanding that explanation.

the tube runs from the point shown into the interior next to the dash and up the post to the headliner and then to the back.

Theres no where for water to contact this tube except for the tip shown and it points downward. So I cant see it being used to divert water.

Not to divert this thread, but you seriously need to lube your door pins.

Just a guess here as I don't have anything like that on my '97.
Does your rear window washer fluid sprayer work?

(I think that tube is too large for the washer solution, but is there any other piece of tubing in the general vacinity that may have become disconnected)

Just a door channel drain, and if GM redirected it down further it wouldnt rust your pins like that, but hit em with a white lithium grease and the water wont wash it away like it does other lubricants.

Experiment : pour a cup of water into the grate where the windsheild wipers are . and in the sunroof track(if you have one ) . Most cars channel the water under the fender . Some with air filtering systems on newer ones are different . Posibly put in for the optional sunroof the dealer would install . Rob

For the Sunroof.

For the Sunroof.

You got a point there, that could very well what it is for.