How does the Rallye Dash Tach hookup?

You could do what you propose, I would NOT use a 20A fuse. That tach needs fractions of an amp. You will unlkely (maybe at Radio Shack) be able to buy very small 3AG fuses, say 2 1/2 or 5A

ANOTHER WAY you could have done this would be to hook a female QC to the spade at bottom right. This would give you "switched" power, then put a universal inline fuse holder in that line and off to the tach.

The whole problem is, 'these old girls' are TERRIBLY under fused.