There's an excellent article on the whole process in the DIY section under 'climate control' . I don't know how to direct you with a link but if you'll check it out it will take you step by step through the whole procedure.

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John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln