heater problems

I have a 1993 bonneville that heat will only come out the right defrost and right front vents, cold air blows out the left side defrost, the left front vents and the floor. I have the manual air conditioning system. A mechanic checked it out and the controls are working, he thinks it is in the air distributor which means pull the dash and wants $700 plus to do it. How hard of a job is it to take the dash out, i am thinking of doing it myself. Thanks for any advice.