I have a few of these with one side wired in. Photo with example. 
 

How can I confirm their current rating?

 

Can I use one side of a piggy back fuse to get power or where do I get the pins so I can pin a fuse and output into the fuse box?

 

It all depends on what you want to. The fuses are grouped and supplied by rear busbars in some cases.

The main choices are 1) how much current do you want to take?  2) Do you want it to be live only when the ignition is on? or 3)  to be live all the time.

If the later (say for a camera) fuse 24 is the 25A supply to the cigarette lighter/12V supply sockets. The fuse you place in the piggy back fuseholder must be no larger than that needed to protect the secondary cable from fire under short circuit conditions. Use the smallest fuse you can and it must be less than the current rating of the connecting wire/cable.

 

You can wire into the back of the fuse holder using a cable of the the correct current rating and with the correct crimped on connector for the fuse. Unless you are experienced in electrical wiring and regulations, I suggest you use the piggy back type fuseholder and a fuse size as discussed above.

 

Choose a fuse location to piggy back onto by selecting one that is rated slightly greater than the sum of the existing load and the extra current. Most things we add to our cars don't take as must current as their paperwork says and you can piggy back on a 5A or 10A fuse without it blowing with the additional load.

 

If you look in the car handbook where it shows fuse rating for optional extras you do not have. You can add a fuse to your piggy back up to the rating shown as the normal fuse in that empty fuse location. But make sure that your on-going wire/cable is capable of carrying that current or down-size the fuse to the cable current rating. 

Cheers!

 

Am I able to access the rear busbar and connect on that? I could in my old TT as it had nuts for the cables coming off it. 
 

Does anyone know the part number or source of the connectors into the fuse box?

 

needs 15 - 20 amp ideally but don’t want to run a cable from the battery if it can be avoided. 

You should run a cable directly from the battery but instead use one of the spare fuse locations in the battery fusebox near the battery.

You should also not do as you did on the TT as the short cable from the busbar  to your next fuse is unprotected against shorts. Contrary to popular belief, fuses are for protection of the cables against overload and resultant fire.