Fuse 5 (the "spare") - Can I employ it?

[Thirstyman]

Has any brave soul figured out how to access the "spare" blade fuse in position 5?

Background:

I bought a Sat Nav from Ebay, which came with a large Cig Lighter SMPS power supply terminated with a micro USB plug (Type B) to power the thing. First I have no Cig Lighter socket, and second, I don't want to look at an ugly umbilical snaking up and over and around even if I did have a socket.

The SMPS draws 38mA with no load, which is too much of a drain on my dinky leisure battery to be permanently wired (under the clock pod for example) so I have built a simple linear regulator (like the one on the clocks' foil circuit) using a LM7805 regulator.

It's very good - less than 3mA with no load, 500mA charging the Sat Nav - so I have it glued with thermal transfer adhesive (Loctite "Output") to the inside of the dash end panel, as it does get rather hot without a heatsink.

I now need to settle on a suitable fused supply, with a view to future expension/hook-up of any more non-essential devices I may want.

Help Please:

At only 500mA full load, I could tap into several circuits that are local to the dash but tantalisingly, there was a 15A blue fuse in "spare" position 5. I removed it for a week to see what happened (or what didn't happen, more accurately), and everything is fine without it.

My question is, how on earth do I access downstream of this fuse? One contact is at battery potential (presumably bussed with the other fuses) but a look at the reverse of the fuseboard gives me no clues as to where fuse 5 circuit might emerge from the board. I bought the Bentley "Official Factory Repair Manual", hoping it may get more intimate than the Haynes, but it doesn't.

So.... has anyone usefully employed this spare fuse #5?

Thanks in advance!