Need advice - won't start
 

My 98 W210 (E300d) won't start. With key turned in ignition, doesn't turn over. (Checked starter relay and starter - both OK, can manually activate starter). Found some dampness in the compartment where the computers located (Tampa where humidity is a constant 80-100%) - dried it out. Main battery (under seat) fully charged, plus new battery in key fob. Need advice on how to troubleshoot without having to tow it to the dealer.