Wyn,
I have a '66 GT, although my heater is a Dometic. However, it sounds as
though you have a passive or 'gravity' heater without a blower or fan to
move the heated air. If so, it is probably very much the same basic design
as mine. The screws holding the outside vent cover have to removed and the
cover pulled off. The cover has two parts to it really. The inner exhaust
pipe runs along inside the fresh air intake pipe, and so the cover has the
job of keeping the two openings seperate. Once the cover is off you should
be able to simply reach into the opening (from the outside), grab the
smaller exhaust pipe and pull as you twist and if it is not rusted too badly
it should slide out. Then from the inside pull the furnace away from the
cabinet and the fresh air intake tube will pull away -- either staying in
the wall or with the furnace. Inspect the exhaust pipe for debris and check
that rust hasn't eaten a hole through it. It is hard to check the baffles in
the furnace for rust but it can be done. The exhaust tube will most likely
be most afflicted with corrosion, but it is the same diameter as automotive
exhaust tubing and you can use that, cut to size, to replace it.
If it is not badly corroded there is a definite advantage to keeping it. It
doesn't require any battery power to run. However, if you have a portable
fan, either 12 volt or 110v, the efficiency can be greatly improved by
aiming the fan at the base of the furnace. That encourages air flow up and
out the top. There is no mixing of outside air or combustion air with
interior air with this type of furnace since it is ducted, and that is an
advantage over the small propane space heaters some like to use. Being
ducted you don't have to crack a window for fresh air.
If the furnace itself looks like it could use a coat of paint, don't be
tempted to use even high-temp spray paint. It will smoke like the dickens,
setting off smoke alarms. Instead, have it powder coated. I had mine done
that way and initially there was just a little smoke, but now the furnace
burns clean. The real caution on older heaters is not just corrosion, but
the regulator. Check it closely for propane leaks as the gaskets can shrink
or crack over the years especially if the furnace has been used a lot. A
replacement regulator would be hard to come by, but a professional can
rebuild them. I was lucky, I was able to locate NOS on E-Bay.
Hope this has been a help to you.
Forrest