It took me all day, but it's done!
Ran the 7 strand into the boot through the drain grommets in the wheel well, then under the hard plastic trim around the lock to the passenger side wing.
Found the correct wires using a multimeter, then spliced in with wago connectors - now I know it works, I'll splice them properly with solder+heat shrink.
Ran a fused 15a supply from the battery (under the bonnet) under the car and into the boot through the same grommet - had to use 2.5mm green+yellow mains earth cable because I couldn't find my reel of red 😞
ill update this thread with the cable colours later.