Chris;

Remember, when in series and passing current, be it normal operating or simply test current, the weakest link will heat and fuse first!...so any fuse value higher than that drawn by the various loads you'd be testing, and total current you'd be drawing, during your check-out (one at a time), would do it...but a 10 or 20A would be good to protect even the smallest gauge used in the car (18Ga?)...and if you can't find that fuseholder you remember having (SOMEWHERE!), simply use a smaller gauge (22Ga) or even a few strands of wire from a heavier gauge wire...this fuse "or frangible link" is really just a last ditch safety measure that you don't expect will blow anyway during testing.

As far as upstream or downstream...you can't get any more upstream than at the Battery...THE energy source, when engine (CHG SYS) is off-line.

Good Hunting!