Fused 10A load from battery, only when car in ready mode - - bad idea?

A 10A fused circuit activated by a 60A rely, toggled by accessory power would be better - automatic cut-off built-in. This way the circuit automatically cuts itself OFF when you turn-off the car.

 

You are thinking about it the right way.

The DC/DC converter is carrying the load. Between the DC/DC converter at the front of the car and the battery at the back of the car, there is a fat wire with a fuse at each end. The fuse at the front end is rated 125 amps and buried insanely deep in the front fuse box. The fuse at the battery end is rated 140 amps and built into the cable end that clamps onto the positive battery post. It's in the part of the clamp that slumps down over the side of the battery like a Dalí watch.

Because the power you're using is coming from up front, if you tap right at the battery, there are two fuse voltage drops between the power source and your tap.

Just as easy is to put a ring terminal on your tap wire and attach it at the lower end (the cable end) of that 140-amp fuse. Now you have two reasons to smile: (1) there is only one fuse voltage drop between either source of the power and your tap, and (2) there is a fuse between your tap and the power source at each end.

You still need your tap to include a fuse holder of its own, with a fuse sized appropriately for the wire and equipment you're connecting. The 125- and 140-amp ratings of the existing fuses won't protect your added wire (unless you add wire fat enough to carry that). But those fuses will protect your added fuse, in the event of an actual short somewhere.

That corner in my car looks kinda like this:



The added fuseholders you see derive from one fat added cable (that you can't really see in the pic) that attaches at the bottom end of that drapey red battery clamp, loops down and up behind the battery to where the added fuseholders are.

Some of the things attached back there are an air compressor that can draw 23 amps and an inverter that can draw 85 or so, depending on what I plug into it. Your ten-amp accessory is no problem, as long as you size your wire and fuse accordingly.