I have a 1997 B2500 Cargo van. It is stuck in defrost mode only. Under the hood on the driver's side there is a rubber grommet that goes thru the firewall that has 4 vac lines that go under the dash above the gas pedal, into a harness, then up to the Climate Control box. Problem is two of the vac lines are rotted right at the grommet. The grommet is molded, therefore the lines don't come out of it. First, what size is the vac line? It is tiny, and I've been to several part stores and they look at me like I am crazy. Next problem, when I do find the correct hose, is how to splice it and reconnect everything. The inner diameter is so small that I do not know if they make a conventional "splice Connector" for this. Anyone else have this problem, and if so what is the best way to fix it?
Thanks!!

Poke a hole through the grommet and run a new vacuum line.

I know that. I can bypass the grommet, but problem is how do I splice into the existing line at the connector, and at the opposite end? I would have to use some kind of butt connector that would be that size, or find some kind of adapter that would go from the small size to a larger size vac line. Only solution I have found so far is buying the replacement harness/line and replacing the whole setup from the back of the climate control to the 4 parts that it connects to. 2 of them are easy to get to, but the other two lines go behind/into the AC box that is under the hood on the passenger side.

Take a piece of the vacuum hose and go to your local hardware store. Most of them have a plumbing section that has little brass fittings for connecting vinyl tubing together. Those work real well on vacuum lines.

try finding rubber tubing that will fit tightly OVER the existing tubing. Do not use a butt connector. Maybe use a piece of writng pen cartridge, cut off a piece to slip into the small tubing to keep it from crushing, then slip the new larger tubing over the whole thing and secure with a small zip tie. Maybe a little silicone grease to seal the larger tubing to the smaller tubing.

i've rigged a patch with heat shrink tubing, glue on the ends, and zip ties on either end for additional strength.

as far as i can tell it works well.