A/C controls

1989 260E,
the air recirculating switch on the dash has no more effect.
Sometime, at full A/C (which is very cold) full heat would suddently come out of all dash vents for a minute or two, than would switch back to full A/C without changing the temp. setting..
I suspect some problems with some vacuum activated traps under the dash. Which one?
Do I have the right diagnostic?
How do you get to those valves?
Thanks
JackD