I do not know that much about the ins and outs of the R129 SL (didn't have the time yet to tear down the car), but I never heard of a third fuse box.
My W124 300CE conv. has the additional electrical devices (relays and compu's) under the rear seat, so I can imagine that the R129 has got them under the front seat or rear deck as well (there are some electrical devices under the rear deck).
Does your top react to the switch or is there absolute no reaction at all?? Do you have the hard or soft top on?
The tranny fluid level can be checked with the dip-stick under the hood (check with running engine and tranny shifted in neutral).
with a minor 600+k