For starters you may want to inspect the pins on the P6 / J6 connectors for corrosion. The N, L1, L2 & L3 signals come thru those connectors. Specifically pins 13, 14, 15, 16, 17 & 18.
Here are the paths for L0, L1, L2 & L3 from S8 to S6 the AM-VM switch
S8 L0 ------ TB4-4------P6-13 &...