Did an easy fix for this on my Mk2 Fabia. I went into the fuse box and found that the fuse for the heated seats was switched by the ignition (or at least the slot was, my Fabia isn’t fitted with such luxuries). According to the owners manual this usually takes a 20A fuse, so I figure that should be plenty safe to run the 12V socket which runs on a 15A fuse as standard.
I then went and picked up a fuse tap, a breakout fuse, and a some bullet connectors. I broke off the inside of the breakout fuse with a screwdriver, connected the fuse tap to the breakout fuse with a bullet connector, and installed the original 15A fuse for the 12V socket into the breakout circuit on the fuse tap.
I then plugged the fuse tap into the heated seat fuse slot, and the breakout fuse into the 12V socket fuse slot, ensuring that the negative of the 12V socket slot was connected to the positive of the heated seat slot.
Works a treat, everything is still fused as it should be, and it's easily reversible. And I definitely went back and shortened those wires instead of just cramming them behind the fuse board cover...