... just take a feed from the cigarette lighter.
Personally, I wouldn't.
1. On a P1, the seats are then permanently live. If the heater element(s) get left on, you've a dead battery a few hours later.
2. Between the heaters and motors, the seats are protected by two fuses ( 20A & 30A respectively ). The cigarette lighter circuit fuse is shared with other systems and is only rated at 20A.
3. Blowing that fuse on a P1 will cause the brake lights to fail.
A much better method -
P1 GTV. If the car doesn't have a sunroof, stick a 30A relay in the relevant socket on the aux relay bracket and pick up a 30A ignition controlled supply at the socket under the radio. If the car does have a sunroof, the same supply can be used - just don't operate the seats at the same time as the sunroof! Dead easy and a good solution on a P1 ( although still not ideal as it still won't allow the seats to be adjusted with the ignition off and when the courtesy lights are still on ). A similar solution could be utilised for a Spider with a manual hood ( but I've not looked at that fully ).
P1 Spider & P2/3 ( Spider/GTV ).
I'd stick a new fuse and relay under the rear parcel shelf. Energise this relay using the same energising supply as the ignition cut off relay ( also under the parcel shelf ). This means the relay will only be on when the ignition key is at 'MAR'. Run the supply wire from this relay down the centre of the back seats, past the airbag ECU and then split it to the two seats. Same limitation as the P1 GTV above ( cannot operate the seat motors with the ignition off ).
Personally, I'd replicate the factory wiring system in it's entirety.