The 5/8 inch hose tightens up to the rear seat heater outlets, just
> fine.The 5/8 inch hose will have a problem passing throught the rubber
> gromets that go between in the metal body between the engine compartment
> and seat area.Get some soap and start struggeling,and they will go
> through.
>

I'm not sure if it's something they did when they went to plastic tees, or
something that came later, but the hoses from the rear core to the plastic
tees on my '90 were 5/8"-sized at the heater core, but tapered down to
something significantly smaller at the tees. The straight parts of the tees
were 5/8, but the side tap was definitely smaller. Perhaps they did that to
increase water flow to the front core, where people want the heat more. In
any case, I tossed the plastic tees and went with some fancy ceramic-coated
Arctic Cat 5/8" tees I found locally (in SoCal!) because I decided to remove
my rear heater entirely and use its plumbing points for the ATF cooler
(saving me big $$$ on two coolant hoses because I could get the "normal"
manual ones vs. the automatic "frankenhoses").

One mild irritation I found with the 5/8 hose I used was that its outside
diameter, over the flared part of any barb fitting I put it on, was just
slightly larger than the VW spring-type hose clamps at full open position. I
had to dremel off about 5mm of the "stop tab" on every one of 'em to get
them to fit over the hose easily. Perhaps the VW hose is was thin-walled, or
the "insulated" type hose I bought was just extra thick. Whatever the issue,
few things are as frustrating as having to pull the hose off to get the
clamp off to dremel the tab down to open the clamp enough to get it to
fit...
--
John Bange
'90 Vanagon - "Geldsauger"