ssaylor,

The blue vacuum lines go to an electric controlled switch under the (left side of the) rear seats.
If this switch is broken, the seats will not be locked either (since you said both seats stopped locking simultaneously...).
This is easy to check (the rear seat can be taken out without any tools).

greetingz,

__________________
1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)