The way piggybacks work is you put the original fuse in the bottom holder to make the original circuit and your new fuse in the top holder to make the wire of the piggyback live, if your not putting the original fuse in the bottom holder then the original circuit cannot go live, plus you should NOT use the 20a fuse for a dash cam! You should find out what current is drawn when the dash cam is in use and use a fuse rated at the next amp above that (if it draws 0.5a use a 1a, if it draws 1a use a 2a etc) as a fuse is there to protect the wiring, even though the piggyback is rated at 20a the wire your connecting in it might not be (defenatly in the case of a dash cam, phone charger etc) this is a problem I see with the picture of the fuse box, i cannot see any fuses in the wires pushed under the fuses, if the wires are lower current than the fuses and something goes wrong with the box and it draws more current than the wires can handle but less than the fuse rating then the wiring will melt and maybe set fire, plus the fact if the unit does go short then the vehicles fuse will blow and take out the circuit on the car (hope the fuses are not for a safety feature!) this scares me as it was supposed to be a professional installation!!!!