I would start by checking the hose that directs air across the temp sensor. I know exactly where this hose is on other models but I'm not familiar with the 126 cars. Hopefully a 126 person will chime in and offer more details.

If the foam hose is gone, which is likely because the disintegrate over time, you are not getting a good air sample across the sensor which will cause eratic Climate Control operation.

Good luck,