Check that R2 is definitely 220R. This is in the path of the FM MPX signal. Also check that R16 on the lower board was jumpered across with a wire. This links the MPX signal to the max4544 switch IC. Check that there is 9V on pin 7 of IC7, the CA3140. Check that R14, R15, and R24 are 200K, 100K, and 10K respectively.

If there is no problem with any of the above, I'm thinking it might be an antenna mismatch/impedance problem. Car aeriels are a compromise at best, and are never very good from an RF standpoint. The one in my Civic (quite an old one) actually made the reception worse when plugged into the tuner prototype. This was traced to a faulty earth on the antenna, which screwed up the ground plane. Replacing it fixed the problem.

pca