I normally dislike white cars, but on some it works well. That's a beauty.
As for the hissing in the car, check for if your car has automatic parking brake release (you'll be able to tell if your car has no Parking brake lever on the lower dash panel). The vacuum valve for automatic parking brake release is often the cause for interior vacuum leaks on these cars. The valve is at the base of the steering column.
The Autotemp II is heavily reliant on vacuum too, so that could also be the culprit.