British wiring convention was that brown wires were always hot and unfused.
I'm not quite following your comment about no connectors on the brown wires and just disappearing back into the bulkhead.
Here is a picture of my series V starter solenoid before I modified my wiring to add an ammeter. I believe this was the stock wiring setup when not having an ammeter.
1. White / green wire that goes to the ignition coil + connection.
2. White / red wire that goes to the ignition switch.
3. Large brown wire that goes to the alternator B+ connection.
4. Two brown wires that are connected into a single spade connection on the starter solenoid. One wire goes to the fuse box terminal 1. The other wire goes to the alternator relay.
So for #4, those wires go into the wiring harness, but don't go back through the firewall.
It sounds like you have a short that is blowing the fuse. The fuse closest to the engine is not used on a series 5 ST. It is used on a GT, but I don't think series 5 GTs were sent to the USA or Canada. Do you have any idea what is connected to that fuse? Stock fuse box setup below.
Mike