It's been a while, but as I recall the 100% recirc modification on the 123 involves the fresh air door, not the recirc door. As designed, the fresh air door is open whenever the climate control is switched on. The recirc door selects either 100% fresh air or 80/20, depending on which position it is in. The modification involves closing the fresh air door whenever the recirc door is in the recirculate position.

I wouldn't worry too much about fresh air in the 20 year old car. A car that age has plenty of air leaks from tired, old, shrunken gaskets and seals.

I measured the performance differnce the change made, so I can state objectively the effect it had. (I keep notes on this stuff.) On a hot Dallas day (105F) it reduced the duct temperature from 58F to 53F. That is enough of a change to be easily noticable; it makes a significant increase in comfort.

Perhaps you don't live in a hot enough climate to require such a change, or perhaps you misunderstood how to perform the modification.

- JimY