When using piggy back fuses you should avoid using any fuse slot that is related to safety or would prevent the car from being driven should any problems arise.
Its a good idea to buy a basic multimeter. They have so many uses around the home and in the car car you will soon wonder how you ever managed without one. £8 -£9 in Halfords are ok and you can find the same design cheaper still on line.
you dont need any great electrical knowledge to use some functions .
0-20 volts DC range will allow you to check whether aa and aaa batteries really need replacement, the state of charge of car batteries, alternator efficiency, whether fuses are constant or ignition (they have 2 tiny brass contacts exposed on the top so you can test for voltage without removal ) etc etc
Ohms resistance range allows you to check for continuity to find broken wires, poor electrical contacts , blown fuses, blown light bulbs, find good earthing points etc
I have saved many pounds over the years by detecting and fixing poor contacts in battery consumer goods, such as torches, remote controls, wireless mouse, etc etc. which might have otherwise been thrown away , or had batteries replaced unnecessarily.