92 Silverado heat-control adjustment

[copco]

I've got a 92 Silverado and two years ago replaced one of the linkage arms on the heat/defrost/vent doors. Since then the thing hasn't gone to full defrost. Is there a particular method of adjusting the blasted thing or is it just hit-and-miss? The linkage just fell off two days ago and now I get to start over. Feels like it's stuck in the vent and partial defrost mode. Blower motor switch works ok.
Thanx for any help.
copco