He was lying to you. Period. He just want a sucker to pay him to fix that
job. The system is controlled by a series of flaps that control amount of
heat to the passenger or driver side. They are called tempering flaps.
I am assuming you set temperature equally on the control... and still have
this problem. It can be cause by bad tempering flap... or bad temperature
I strongly suggest you let the dealer fix this job for you... they got the
technical support there. Or if you want to fix it yourself... get yourself a
factory service manual... which reminds me... I need to get one too... for