The thermal valve is in the manifold. Did it have that number? The parts info shows the amp item 90 as the number you gave. If in error the info at least has us on the same system then the amplifier (item 90) should sit beside the distributor if I remember correctly. The diagram shows how it is plumbed. The EGR valve is on the left manifold and is represented in the diagram in about the right position (item 89).
Bosch Master, ASE Master, L1
33 years MB technician